From 9fb9e817012441120d857b5d496185deb5e63309 Mon Sep 17 00:00:00 2001
From: rbalsleyMSFT <53497092+rbalsleyMSFT@users.noreply.github.com>
Date: Fri, 5 Jun 2026 09:46:00 -0700
Subject: [PATCH] Add configurable FFU build partition drive letters

Add System, Windows, and Recovery partition drive-letter settings to the Hyper-V Settings UI, config save/load, BuildFFUVM parameters, and sample/docs. Defaults remain S, W, and R.

Validate selected letters early, log validation failures for the UI, include drive-letter metadata in VHDX cache matching, and normalize legacy malformed cache values. This only affects FFU build-time partitioning; ApplyFFU deployment letters remain unchanged.
---
 FFUDevelopment/BuildFFUVM.ps1                 | 124 ++++++++++++++++--
 FFUDevelopment/BuildFFUVM_UI.ps1              |  34 +++--
 FFUDevelopment/BuildFFUVM_UI.xaml             |  90 +++++++++++++
 .../FFUUI.Core/FFUUI.Core.Config.psm1         |   6 +
 .../FFUUI.Core/FFUUI.Core.Initialize.psm1     |   6 +
 FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1     |   3 +
 FFUDevelopment/config/Sample_default.json     | Bin 7042 -> 7288 bytes
 docs/appsscriptvariables.md                   |   3 +
 docs/hyperv_settings.md                       |  14 ++
 docs/parameters_reference.md                  |   3 +
 10 files changed, 262 insertions(+), 21 deletions(-)

diff --git a/FFUDevelopment/BuildFFUVM.ps1 b/FFUDevelopment/BuildFFUVM.ps1
index 2d9267d..5704935 100644
--- a/FFUDevelopment/BuildFFUVM.ps1
+++ b/FFUDevelopment/BuildFFUVM.ps1
@@ -147,6 +147,15 @@ Path to the Windows 10/11 ISO file.
 .PARAMETER LogicalSectorSizeBytes
 UInt32 value of 512 or 4096. Useful for 4Kn drives or devices shipping with UFS drives. Default is 512.
 
+.PARAMETER SystemPartitionDriveLetter
+Drive letter used for the System partition while building the FFU VHDX. Default is S.
+
+.PARAMETER WindowsPartitionDriveLetter
+Drive letter used for the Windows partition while building the FFU VHDX. Default is W.
+
+.PARAMETER RecoveryPartitionDriveLetter
+Drive letter used for the Recovery partition while building the FFU VHDX. Default is R.
+
 .PARAMETER Make
 Make of the device to download drivers. Accepted values are: 'Microsoft', 'Dell', 'HP', 'Lenovo'.
 
@@ -411,6 +420,9 @@ param(
     [string]$MediaType = 'consumer',
     [ValidateSet(512, 4096)]
     [uint32]$LogicalSectorSizeBytes = 512,
+    [string]$SystemPartitionDriveLetter = 'S',
+    [string]$WindowsPartitionDriveLetter = 'W',
+    [string]$RecoveryPartitionDriveLetter = 'R',
     [bool]$Optimize = $true,
     [string]$DriversJsonPath,
     [bool]$CompressDownloadedDriversToWim = $false,
@@ -866,6 +878,9 @@ class VhdxCacheItem {
     [string]$VhdxFileName = ""
     [uint32]$LogicalSectorSizeBytes = ""
     [uint64]$Disksize = ""
+    [string]$SystemPartitionDriveLetter = ""
+    [string]$WindowsPartitionDriveLetter = ""
+    [string]$RecoveryPartitionDriveLetter = ""
     [string]$WindowsSKU = ""
     [string]$WindowsRelease = ""
     [string]$WindowsVersion = ""
@@ -3048,21 +3063,80 @@ function New-ScratchVhdx {
     Writelog "Done."
     return $toReturn
 }
+function Get-NormalizedPartitionDriveLetters {
+    param(
+        [string]$SystemPartitionDriveLetter,
+        [string]$WindowsPartitionDriveLetter,
+        [string]$RecoveryPartitionDriveLetter,
+        [switch]$ValidateAvailable
+    )
+
+    $requestedLetters = [ordered]@{
+        SystemPartitionDriveLetter   = $SystemPartitionDriveLetter
+        WindowsPartitionDriveLetter  = $WindowsPartitionDriveLetter
+        RecoveryPartitionDriveLetter = $RecoveryPartitionDriveLetter
+    }
+    $normalizedLetters = [ordered]@{}
+
+    foreach ($entry in $requestedLetters.GetEnumerator()) {
+        $driveLetter = ([string]$entry.Value).Trim().TrimEnd(':').ToUpperInvariant()
+        if ([string]::IsNullOrWhiteSpace($driveLetter) -or $driveLetter -notmatch '^[A-Z]$') {
+            throw "$($entry.Key) must be a single drive letter from A to Z without a colon."
+        }
+
+        $normalizedLetters[$entry.Key] = $driveLetter
+    }
+
+    $duplicateLetters = @($normalizedLetters.Values | Group-Object | Where-Object { $_.Count -gt 1 })
+    if ($duplicateLetters.Count -gt 0) {
+        $duplicateLetterList = ($duplicateLetters | ForEach-Object { $_.Name }) -join ', '
+        throw "System, Windows, and Recovery partition drive letters must be unique. Duplicate value(s): $duplicateLetterList."
+    }
+
+    if ($ValidateAvailable) {
+        foreach ($entry in $normalizedLetters.GetEnumerator()) {
+            $existingDrive = Get-PSDrive -Name $entry.Value -PSProvider FileSystem -ErrorAction SilentlyContinue
+            if ($null -ne $existingDrive) {
+                throw "$($entry.Key) uses drive letter $($entry.Value), but that letter is already assigned on this host. Choose an unused drive letter."
+            }
+        }
+    }
+
+    return [pscustomobject]$normalizedLetters
+}
+function Get-PartitionDriveLetterCacheValue {
+    param(
+        [object]$DriveLetterValue
+    )
+
+    $driveLetter = ([string]$DriveLetterValue).Trim().TrimEnd(':').ToUpperInvariant()
+    if ($driveLetter -match '^[A-Z]$') {
+        return $driveLetter
+    }
+
+    $trailingDriveLetter = [regex]::Match($driveLetter, '(?i)(?:^|[^A-Z])([A-Z])$')
+    if ($trailingDriveLetter.Success) {
+        return $trailingDriveLetter.Groups[1].Value.ToUpperInvariant()
+    }
+
+    return $driveLetter
+}
 #Add System Partition
 function New-SystemPartition {
     param(
         [Parameter(Mandatory = $true)]
         [ciminstance]$VhdxDisk,
+        [string]$DriveLetter = 'S',
         [uint64]$SystemPartitionSize = 260MB
     )
 
     WriteLog "Creating System partition..."
 
-    $sysPartition = $VhdxDisk | New-Partition -DriveLetter 'S' -Size $SystemPartitionSize -GptType "{c12a7328-f81f-11d2-ba4b-00a0c93ec93b}" -IsHidden 
-    $sysPartition | Format-Volume -FileSystem FAT32 -Force -NewFileSystemLabel "System" 
+    $sysPartition = $VhdxDisk | New-Partition -DriveLetter $DriveLetter -Size $SystemPartitionSize -GptType "{c12a7328-f81f-11d2-ba4b-00a0c93ec93b}" -IsHidden 
+    $sysPartition | Format-Volume -FileSystem FAT32 -Force -NewFileSystemLabel "System" | Out-Null
 
     WriteLog 'Done.'
-    return $sysPartition.DriveLetter
+    return [string]$sysPartition.DriveLetter
 }
 #Add MSRPartition
 function New-MSRPartition {
@@ -3088,16 +3162,17 @@ function New-OSPartition {
         [Parameter(Mandatory = $true)]
         [string]$WimPath,
         [uint32]$WimIndex,
+        [string]$DriveLetter = 'W',
         [uint64]$OSPartitionSize = 0
     )
 
     WriteLog "Creating OS partition..."
 
     if ($OSPartitionSize -gt 0) {
-        $osPartition = $vhdxDisk | New-Partition -DriveLetter 'W' -Size $OSPartitionSize -GptType "{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}" 
+        $osPartition = $vhdxDisk | New-Partition -DriveLetter $DriveLetter -Size $OSPartitionSize -GptType "{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}" 
     }
     else {
-        $osPartition = $vhdxDisk | New-Partition -DriveLetter 'W' -UseMaximumSize -GptType "{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}" 
+        $osPartition = $vhdxDisk | New-Partition -DriveLetter $DriveLetter -UseMaximumSize -GptType "{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}" 
     }
 
     $osPartition | Format-Volume -FileSystem NTFS -Confirm:$false -Force -NewFileSystemLabel "Windows" 
@@ -3129,6 +3204,7 @@ function New-RecoveryPartition {
         [Parameter(Mandatory = $true)]
         $OsPartition,
         [uint64]$RecoveryPartitionSize = 0,
+        [string]$DriveLetter = 'R',
         [ciminstance]$DataPartition
     )
 
@@ -3163,7 +3239,7 @@ function New-RecoveryPartition {
                 WriteLog "OS partition shrunk by $calculatedRecoverySize bytes for Recovery partition."
             }
 
-            $recoveryPartition = $VhdxDisk | New-Partition -DriveLetter 'R' -UseMaximumSize -GptType "{de94bba4-06d1-4d40-a16a-bfd50179d6ac}" `
+            $recoveryPartition = $VhdxDisk | New-Partition -DriveLetter $DriveLetter -UseMaximumSize -GptType "{de94bba4-06d1-4d40-a16a-bfd50179d6ac}" `
             | Format-Volume -FileSystem NTFS -Confirm:$false -Force -NewFileSystemLabel 'Recovery' 
 
             WriteLog "Done. Recovery partition at drive $($recoveryPartition.DriveLetter):"
@@ -5877,7 +5953,6 @@ if ($ExportConfigFile) {
 
 ####### End Generate Config File #######
 
-
 #Setting long path support - this prevents issues where some applications have deep directory structures
 #and oscdimg fails to create the Apps ISO
 try {
@@ -5896,6 +5971,21 @@ if ($LongPathsEnabled -ne 1) {
 Set-Progress -Percentage 2 -Message "Validating parameters..."
 ###PARAMETER VALIDATION
 
+#Set build partition drive letters and validate they are available for use; this is required before any build steps that require drive access to ensure the expected drive letters are reserved and to fail fast if there are conflicts.
+try {
+    $partitionDriveLetters = Get-NormalizedPartitionDriveLetters -SystemPartitionDriveLetter $SystemPartitionDriveLetter -WindowsPartitionDriveLetter $WindowsPartitionDriveLetter -RecoveryPartitionDriveLetter $RecoveryPartitionDriveLetter -ValidateAvailable
+    $SystemPartitionDriveLetter = $partitionDriveLetters.SystemPartitionDriveLetter
+    $WindowsPartitionDriveLetter = $partitionDriveLetters.WindowsPartitionDriveLetter
+    $RecoveryPartitionDriveLetter = $partitionDriveLetters.RecoveryPartitionDriveLetter
+    WriteLog "Using build partition drive letters: System=$SystemPartitionDriveLetter, Windows=$WindowsPartitionDriveLetter, Recovery=$RecoveryPartitionDriveLetter"
+}
+catch {
+    $partitionDriveLetterValidationError = "Build validation failed: $($_.Exception.Message)"
+    Set-Progress -Percentage 2 -Message $partitionDriveLetterValidationError
+    WriteLog $partitionDriveLetterValidationError
+    throw $partitionDriveLetterValidationError
+}
+
 #Validate CopyDrivers dependency on BuildUSBDrive
 if ($CopyDrivers -and (-not $BuildUSBDrive)) {
     WriteLog "-CopyDrivers is set to `$true, but -BuildUSBDrive is not set to `$true"
@@ -7187,6 +7277,15 @@ try {
                     [uint64]$cachedDisksize = 0
                     if (-not [uint64]::TryParse([string]$vhdxCacheItem.Disksize, [ref]$cachedDisksize)) { WriteLog "Disksize invalid in cached config ($($vhdxCacheItem.Disksize)), continuing"; continue }
                     if ($cachedDisksize -ne $Disksize) { WriteLog "Disksize mismatch (cached: $cachedDisksize, current: $Disksize), continuing"; continue }
+                    if ($vhdxCacheItem.PSObject.Properties.Name -notcontains 'SystemPartitionDriveLetter') { WriteLog 'SystemPartitionDriveLetter missing in cached config, continuing'; continue }
+                    if ($vhdxCacheItem.PSObject.Properties.Name -notcontains 'WindowsPartitionDriveLetter') { WriteLog 'WindowsPartitionDriveLetter missing in cached config, continuing'; continue }
+                    if ($vhdxCacheItem.PSObject.Properties.Name -notcontains 'RecoveryPartitionDriveLetter') { WriteLog 'RecoveryPartitionDriveLetter missing in cached config, continuing'; continue }
+                    $cachedSystemPartitionDriveLetter = Get-PartitionDriveLetterCacheValue -DriveLetterValue $vhdxCacheItem.SystemPartitionDriveLetter
+                    $cachedWindowsPartitionDriveLetter = Get-PartitionDriveLetterCacheValue -DriveLetterValue $vhdxCacheItem.WindowsPartitionDriveLetter
+                    $cachedRecoveryPartitionDriveLetter = Get-PartitionDriveLetterCacheValue -DriveLetterValue $vhdxCacheItem.RecoveryPartitionDriveLetter
+                    if ($cachedSystemPartitionDriveLetter -ne $SystemPartitionDriveLetter) { WriteLog "SystemPartitionDriveLetter mismatch (cached: $($vhdxCacheItem.SystemPartitionDriveLetter), current: $SystemPartitionDriveLetter), continuing"; continue }
+                    if ($cachedWindowsPartitionDriveLetter -ne $WindowsPartitionDriveLetter) { WriteLog "WindowsPartitionDriveLetter mismatch (cached: $($vhdxCacheItem.WindowsPartitionDriveLetter), current: $WindowsPartitionDriveLetter), continuing"; continue }
+                    if ($cachedRecoveryPartitionDriveLetter -ne $RecoveryPartitionDriveLetter) { WriteLog "RecoveryPartitionDriveLetter mismatch (cached: $($vhdxCacheItem.RecoveryPartitionDriveLetter), current: $RecoveryPartitionDriveLetter), continuing"; continue }
 
                     $cachedUpdateNames = @()
                     if ($vhdxCacheItem.IncludedUpdates -and $vhdxCacheItem.IncludedUpdates.Count -gt 0) {
@@ -7504,21 +7603,21 @@ try {
 
         $vhdxDisk = New-ScratchVhdx -VhdxPath $VHDXPath -SizeBytes $disksize -LogicalSectorSizeBytes $LogicalSectorSizeBytes
 
-        $systemPartitionDriveLetter = New-SystemPartition -VhdxDisk $vhdxDisk
+        $createdSystemPartitionDriveLetter = New-SystemPartition -VhdxDisk $vhdxDisk -DriveLetter $SystemPartitionDriveLetter
     
         New-MSRPartition -VhdxDisk $vhdxDisk
     
         Set-Progress -Percentage 16 -Message "Applying base Windows image to VHDX..."
-        $osPartition = New-OSPartition -VhdxDisk $vhdxDisk -OSPartitionSize $OSPartitionSize -WimPath $WimPath -WimIndex $index
+        $osPartition = New-OSPartition -VhdxDisk $vhdxDisk -OSPartitionSize $OSPartitionSize -WimPath $WimPath -WimIndex $index -DriveLetter $WindowsPartitionDriveLetter
         $osPartitionDriveLetter = $osPartition[1].DriveLetter
         $WindowsPartition = $osPartitionDriveLetter + ':\'
 
         #$recoveryPartition = New-RecoveryPartition -VhdxDisk $vhdxDisk -OsPartition $osPartition[1] -RecoveryPartitionSize $RecoveryPartitionSize -DataPartition $dataPartition
-        $recoveryPartition = New-RecoveryPartition -VhdxDisk $vhdxDisk -OsPartition $osPartition[1] -RecoveryPartitionSize $RecoveryPartitionSize -DataPartition $dataPartition
+        $recoveryPartition = New-RecoveryPartition -VhdxDisk $vhdxDisk -OsPartition $osPartition[1] -RecoveryPartitionSize $RecoveryPartitionSize -DriveLetter $RecoveryPartitionDriveLetter -DataPartition $dataPartition
 
         WriteLog 'All necessary partitions created.'
 
-        Add-BootFiles -OsPartitionDriveLetter $osPartitionDriveLetter -SystemPartitionDriveLetter $systemPartitionDriveLetter[1] -AdkPath $adkPath -WindowsArch $WindowsArch
+        Add-BootFiles -OsPartitionDriveLetter $osPartitionDriveLetter -SystemPartitionDriveLetter $createdSystemPartitionDriveLetter -AdkPath $adkPath -WindowsArch $WindowsArch
     
         #Add Windows packages
         if ($UpdateLatestCU -or $UpdateLatestNet -or $UpdatePreviewCU ) {
@@ -7689,6 +7788,9 @@ try {
         $cachedVHDXInfo.VhdxFileName = $("$VMName.vhdx")
         $cachedVHDXInfo.LogicalSectorSizeBytes = $LogicalSectorSizeBytes
         $cachedVHDXInfo.Disksize = $Disksize
+        $cachedVHDXInfo.SystemPartitionDriveLetter = [string]$SystemPartitionDriveLetter
+        $cachedVHDXInfo.WindowsPartitionDriveLetter = [string]$WindowsPartitionDriveLetter
+        $cachedVHDXInfo.RecoveryPartitionDriveLetter = [string]$RecoveryPartitionDriveLetter
         $cachedVHDXInfo.WindowsSKU = $WindowsSKU
         $cachedVHDXInfo.WindowsRelease = $WindowsRelease
         $cachedVHDXInfo.WindowsVersion = $WindowsVersion
diff --git a/FFUDevelopment/BuildFFUVM_UI.ps1 b/FFUDevelopment/BuildFFUVM_UI.ps1
index ac5de98..6a52864 100644
--- a/FFUDevelopment/BuildFFUVM_UI.ps1
+++ b/FFUDevelopment/BuildFFUVM_UI.ps1
@@ -338,7 +338,6 @@ $script:uiState.Controls.btnRun.Add_Click({
                 $script:uiState.Flags.isCleanupRunning = $true
 
                 $script:uiState.Data.pollTimer.Add_Tick({
-                        param($sender, $e)
                         $currentProcess = $script:uiState.Data.currentBuildProcess
 
                         # Read new lines from log
@@ -353,13 +352,13 @@ $script:uiState.Controls.btnRun.Add_Click({
                         }
 
                         if ($null -eq $currentProcess -or $null -eq $script:uiState.Data.pollTimer) {
-                            if ($null -ne $sender) { $sender.Stop() }
+                            if ($null -ne $script:uiState.Data.pollTimer) { $script:uiState.Data.pollTimer.Stop() }
                             $script:uiState.Data.pollTimer = $null
                             return
                         }
 
                         if ($currentProcess.HasExited) {
-                            if ($null -ne $sender) { $sender.Stop() }
+                            if ($null -ne $script:uiState.Data.pollTimer) { $script:uiState.Data.pollTimer.Stop() }
                             $script:uiState.Data.pollTimer = $null
 
                             if ($null -ne $script:uiState.Data.logStreamReader) {
@@ -621,7 +620,6 @@ $script:uiState.Controls.btnRun.Add_Click({
             
             # Add the Tick event handler
             $script:uiState.Data.pollTimer.Add_Tick({
-                    param($sender, $e)
                     # This scriptblock runs on the UI thread, so it can safely access script-scoped variables
                     $currentProcess = $script:uiState.Data.currentBuildProcess
                     
@@ -649,8 +647,8 @@ $script:uiState.Controls.btnRun.Add_Click({
 
                     # If process is somehow null or the timer has been nulled out, stop the timer
                     if ($null -eq $currentProcess -or $null -eq $script:uiState.Data.pollTimer) {
-                        if ($null -ne $sender) {
-                            $sender.Stop()
+                        if ($null -ne $script:uiState.Data.pollTimer) {
+                            $script:uiState.Data.pollTimer.Stop()
                         }
                         $script:uiState.Data.pollTimer = $null
                         return
@@ -659,8 +657,8 @@ $script:uiState.Controls.btnRun.Add_Click({
                     # Check if the build process has exited
                     if ($currentProcess.HasExited) {
                         # Stop the timer, we're done polling
-                        if ($null -ne $sender) {
-                            $sender.Stop()
+                        if ($null -ne $script:uiState.Data.pollTimer) {
+                            $script:uiState.Data.pollTimer.Stop()
                         }
                         $script:uiState.Data.pollTimer = $null
                         
@@ -698,9 +696,25 @@ $script:uiState.Controls.btnRun.Add_Click({
                         # Determine final status based on process exit code
                         $finalStatusText = "FFU build completed successfully."
                         if ($exitCode -ne 0) {
-                            $finalStatusText = "FFU build failed. Check FFUDevelopment.log for details."
+                            $failureDetail = $null
+                            if ($null -ne $script:uiState.Data.logData) {
+                                for ($logIndex = $script:uiState.Data.logData.Count - 1; $logIndex -ge 0; $logIndex--) {
+                                    $logLine = [string]$script:uiState.Data.logData[$logIndex]
+                                    if ($logLine -match '(?i)(Build validation failed|Exception|ERROR|already assigned|must be a single drive letter|must be unique)') {
+                                        $failureDetail = $logLine
+                                        break
+                                    }
+                                }
+                            }
+
+                            $finalStatusText = if ($failureDetail) { "FFU build failed. $failureDetail" } else { "FFU build failed. Check FFUDevelopment.log for details." }
                             WriteLog "BuildFFUVM.ps1 process failed with exit code: $exitCode"
-                            [System.Windows.MessageBox]::Show("The build process failed. Please check the $FFUDevelopmentPath\FFUDevelopment.log file for details.`n`nExit code: $exitCode", "Build Error", "OK", "Error") | Out-Null
+                            $buildErrorMessage = "The build process failed. Please check the $FFUDevelopmentPath\FFUDevelopment.log file for details."
+                            if ($failureDetail) {
+                                $buildErrorMessage += "`n`n$failureDetail"
+                            }
+                            $buildErrorMessage += "`n`nExit code: $exitCode"
+                            [System.Windows.MessageBox]::Show($buildErrorMessage, "Build Error", "OK", "Error") | Out-Null
                             $script:uiState.Controls.pbOverallProgress.Visibility = 'Collapsed'
                         }
                         else {
diff --git a/FFUDevelopment/BuildFFUVM_UI.xaml b/FFUDevelopment/BuildFFUVM_UI.xaml
index c668923..761270b 100644
--- a/FFUDevelopment/BuildFFUVM_UI.xaml
+++ b/FFUDevelopment/BuildFFUVM_UI.xaml
@@ -339,6 +339,96 @@
                                     <!-- VM Name Prefix -->
                                     <TextBlock Text="VM Name Prefix" Margin="0,0,0,8" ToolTip="Prefix for the VM Name. The default is _FFU."/>
                                     <TextBox x:Name="txtVMNamePrefix" HorizontalAlignment="Stretch" Margin="0,0,0,20" ToolTip="Prefix for the VM Name. The default is _FFU."/>
+                                    <!-- System Partition Drive Letter -->
+                                    <TextBlock Text="System Partition Drive Letter" Margin="0,0,0,8" ToolTip="Drive letter used for the System partition while building the FFU VHDX. Default is S."/>
+                                    <ComboBox x:Name="cmbSystemPartitionDriveLetter" HorizontalAlignment="Left" Margin="0,0,0,20" ToolTip="Drive letter used for the System partition while building the FFU VHDX. Default is S.">
+                                        <ComboBoxItem Content="A"/>
+                                        <ComboBoxItem Content="B"/>
+                                        <ComboBoxItem Content="C"/>
+                                        <ComboBoxItem Content="D"/>
+                                        <ComboBoxItem Content="E"/>
+                                        <ComboBoxItem Content="F"/>
+                                        <ComboBoxItem Content="G"/>
+                                        <ComboBoxItem Content="H"/>
+                                        <ComboBoxItem Content="I"/>
+                                        <ComboBoxItem Content="J"/>
+                                        <ComboBoxItem Content="K"/>
+                                        <ComboBoxItem Content="L"/>
+                                        <ComboBoxItem Content="M"/>
+                                        <ComboBoxItem Content="N"/>
+                                        <ComboBoxItem Content="O"/>
+                                        <ComboBoxItem Content="P"/>
+                                        <ComboBoxItem Content="Q"/>
+                                        <ComboBoxItem Content="R"/>
+                                        <ComboBoxItem Content="S" IsSelected="True"/>
+                                        <ComboBoxItem Content="T"/>
+                                        <ComboBoxItem Content="U"/>
+                                        <ComboBoxItem Content="V"/>
+                                        <ComboBoxItem Content="W"/>
+                                        <ComboBoxItem Content="X"/>
+                                        <ComboBoxItem Content="Y"/>
+                                        <ComboBoxItem Content="Z"/>
+                                    </ComboBox>
+                                    <!-- Windows Partition Drive Letter -->
+                                    <TextBlock Text="Windows Partition Drive Letter" Margin="0,0,0,8" ToolTip="Drive letter used for the Windows partition while building the FFU VHDX. Default is W."/>
+                                    <ComboBox x:Name="cmbWindowsPartitionDriveLetter" HorizontalAlignment="Left" Margin="0,0,0,20" ToolTip="Drive letter used for the Windows partition while building the FFU VHDX. Default is W.">
+                                        <ComboBoxItem Content="A"/>
+                                        <ComboBoxItem Content="B"/>
+                                        <ComboBoxItem Content="C"/>
+                                        <ComboBoxItem Content="D"/>
+                                        <ComboBoxItem Content="E"/>
+                                        <ComboBoxItem Content="F"/>
+                                        <ComboBoxItem Content="G"/>
+                                        <ComboBoxItem Content="H"/>
+                                        <ComboBoxItem Content="I"/>
+                                        <ComboBoxItem Content="J"/>
+                                        <ComboBoxItem Content="K"/>
+                                        <ComboBoxItem Content="L"/>
+                                        <ComboBoxItem Content="M"/>
+                                        <ComboBoxItem Content="N"/>
+                                        <ComboBoxItem Content="O"/>
+                                        <ComboBoxItem Content="P"/>
+                                        <ComboBoxItem Content="Q"/>
+                                        <ComboBoxItem Content="R"/>
+                                        <ComboBoxItem Content="S"/>
+                                        <ComboBoxItem Content="T"/>
+                                        <ComboBoxItem Content="U"/>
+                                        <ComboBoxItem Content="V"/>
+                                        <ComboBoxItem Content="W" IsSelected="True"/>
+                                        <ComboBoxItem Content="X"/>
+                                        <ComboBoxItem Content="Y"/>
+                                        <ComboBoxItem Content="Z"/>
+                                    </ComboBox>
+                                    <!-- Recovery Partition Drive Letter -->
+                                    <TextBlock Text="Recovery Partition Drive Letter" Margin="0,0,0,8" ToolTip="Drive letter used for the Recovery partition while building the FFU VHDX. Default is R."/>
+                                    <ComboBox x:Name="cmbRecoveryPartitionDriveLetter" HorizontalAlignment="Left" Margin="0,0,0,20" ToolTip="Drive letter used for the Recovery partition while building the FFU VHDX. Default is R.">
+                                        <ComboBoxItem Content="A"/>
+                                        <ComboBoxItem Content="B"/>
+                                        <ComboBoxItem Content="C"/>
+                                        <ComboBoxItem Content="D"/>
+                                        <ComboBoxItem Content="E"/>
+                                        <ComboBoxItem Content="F"/>
+                                        <ComboBoxItem Content="G"/>
+                                        <ComboBoxItem Content="H"/>
+                                        <ComboBoxItem Content="I"/>
+                                        <ComboBoxItem Content="J"/>
+                                        <ComboBoxItem Content="K"/>
+                                        <ComboBoxItem Content="L"/>
+                                        <ComboBoxItem Content="M"/>
+                                        <ComboBoxItem Content="N"/>
+                                        <ComboBoxItem Content="O"/>
+                                        <ComboBoxItem Content="P"/>
+                                        <ComboBoxItem Content="Q"/>
+                                        <ComboBoxItem Content="R" IsSelected="True"/>
+                                        <ComboBoxItem Content="S"/>
+                                        <ComboBoxItem Content="T"/>
+                                        <ComboBoxItem Content="U"/>
+                                        <ComboBoxItem Content="V"/>
+                                        <ComboBoxItem Content="W"/>
+                                        <ComboBoxItem Content="X"/>
+                                        <ComboBoxItem Content="Y"/>
+                                        <ComboBoxItem Content="Z"/>
+                                    </ComboBox>
                                     <!-- Logical Sector Size -->
                                     <TextBlock Text="Logical Sector Size" Margin="0,0,0,8" ToolTip="Unit32 value of 512 or 4096. Useful for 4Kn drives or devices shipping with UFS drives. Default is 512."/>
                                     <ComboBox x:Name="cmbLogicalSectorSize" HorizontalAlignment="Left" ToolTip="Unit32 value of 512 or 4096. Useful for 4Kn drives or devices shipping with UFS drives. Default is 512.">
diff --git a/FFUDevelopment/FFUUI.Core/FFUUI.Core.Config.psm1 b/FFUDevelopment/FFUUI.Core/FFUUI.Core.Config.psm1
index 90616a4..2c75d4e 100644
--- a/FFUDevelopment/FFUUI.Core/FFUUI.Core.Config.psm1
+++ b/FFUDevelopment/FFUUI.Core/FFUUI.Core.Config.psm1
@@ -62,6 +62,9 @@ function Get-UIConfig {
         InstallWingetApps              = $State.Controls.chkInstallWingetApps.IsChecked
         ISOPath                        = $State.Controls.txtISOPath.Text
         WindowsMediaSource             = if ($null -ne $State.Controls.rbProvideISO -and $State.Controls.rbProvideISO.IsChecked) { "Provide Windows ISO" } else { "Download Windows ESD" }
+        SystemPartitionDriveLetter     = $State.Controls.cmbSystemPartitionDriveLetter.SelectedItem.Content
+        WindowsPartitionDriveLetter    = $State.Controls.cmbWindowsPartitionDriveLetter.SelectedItem.Content
+        RecoveryPartitionDriveLetter   = $State.Controls.cmbRecoveryPartitionDriveLetter.SelectedItem.Content
         LogicalSectorSizeBytes         = [int]$State.Controls.cmbLogicalSectorSize.SelectedItem.Content
         # Make                           = $null
         MediaType                      = $State.Controls.cmbMediaType.SelectedItem
@@ -523,6 +526,9 @@ function Update-UIFromConfig {
     Set-UIValue -ControlName 'txtProcessors' -PropertyName 'Text' -ConfigObject $ConfigContent -ConfigKey 'Processors' -State $State
     Set-UIValue -ControlName 'txtVMLocation' -PropertyName 'Text' -ConfigObject $ConfigContent -ConfigKey 'VMLocation' -State $State
     Set-UIValue -ControlName 'txtVMNamePrefix' -PropertyName 'Text' -ConfigObject $ConfigContent -ConfigKey 'FFUPrefix' -State $State
+    Set-UIValue -ControlName 'cmbSystemPartitionDriveLetter' -PropertyName 'SelectedItem' -ConfigObject $ConfigContent -ConfigKey 'SystemPartitionDriveLetter' -TransformValue { param($val) ([string]$val).Trim().TrimEnd(':').ToUpperInvariant() } -State $State
+    Set-UIValue -ControlName 'cmbWindowsPartitionDriveLetter' -PropertyName 'SelectedItem' -ConfigObject $ConfigContent -ConfigKey 'WindowsPartitionDriveLetter' -TransformValue { param($val) ([string]$val).Trim().TrimEnd(':').ToUpperInvariant() } -State $State
+    Set-UIValue -ControlName 'cmbRecoveryPartitionDriveLetter' -PropertyName 'SelectedItem' -ConfigObject $ConfigContent -ConfigKey 'RecoveryPartitionDriveLetter' -TransformValue { param($val) ([string]$val).Trim().TrimEnd(':').ToUpperInvariant() } -State $State
     Set-UIValue -ControlName 'cmbLogicalSectorSize' -PropertyName 'SelectedItem' -ConfigObject $ConfigContent -ConfigKey 'LogicalSectorSizeBytes' -TransformValue { param($val) $val.ToString() } -State $State
     $State.Controls.spVMNetworkingSettings.IsEnabled = $true -eq $State.Controls.chkEnableVMNetworking.IsChecked
     if (-not ($true -eq $State.Controls.chkEnableVMNetworking.IsChecked)) {
diff --git a/FFUDevelopment/FFUUI.Core/FFUUI.Core.Initialize.psm1 b/FFUDevelopment/FFUUI.Core/FFUUI.Core.Initialize.psm1
index ca8f396..73fdf74 100644
--- a/FFUDevelopment/FFUUI.Core/FFUUI.Core.Initialize.psm1
+++ b/FFUDevelopment/FFUUI.Core/FFUUI.Core.Initialize.psm1
@@ -255,6 +255,9 @@ function Initialize-UIControls {
     $State.Controls.txtProcessors = $window.FindName('txtProcessors')
     $State.Controls.txtVMLocation = $window.FindName('txtVMLocation')
     $State.Controls.txtVMNamePrefix = $window.FindName('txtVMNamePrefix')
+    $State.Controls.cmbSystemPartitionDriveLetter = $window.FindName('cmbSystemPartitionDriveLetter')
+    $State.Controls.cmbWindowsPartitionDriveLetter = $window.FindName('cmbWindowsPartitionDriveLetter')
+    $State.Controls.cmbRecoveryPartitionDriveLetter = $window.FindName('cmbRecoveryPartitionDriveLetter')
     $State.Controls.cmbLogicalSectorSize = $window.FindName('cmbLogicalSectorSize')
     $State.Controls.txtProductKey = $window.FindName('txtProductKey')
     $State.Controls.txtOfficePath = $window.FindName('txtOfficePath')
@@ -445,6 +448,9 @@ function Initialize-UIDefaults {
     $State.Controls.txtProcessors.Text = $State.Defaults.generalDefaults.Processors
     $State.Controls.txtVMLocation.Text = $State.Defaults.generalDefaults.VMLocation
     $State.Controls.txtVMNamePrefix.Text = $State.Defaults.generalDefaults.VMNamePrefix
+    $State.Controls.cmbSystemPartitionDriveLetter.SelectedItem = ($State.Controls.cmbSystemPartitionDriveLetter.Items | Where-Object { $_.Content -eq $State.Defaults.generalDefaults.SystemPartitionDriveLetter })
+    $State.Controls.cmbWindowsPartitionDriveLetter.SelectedItem = ($State.Controls.cmbWindowsPartitionDriveLetter.Items | Where-Object { $_.Content -eq $State.Defaults.generalDefaults.WindowsPartitionDriveLetter })
+    $State.Controls.cmbRecoveryPartitionDriveLetter.SelectedItem = ($State.Controls.cmbRecoveryPartitionDriveLetter.Items | Where-Object { $_.Content -eq $State.Defaults.generalDefaults.RecoveryPartitionDriveLetter })
     $State.Controls.cmbLogicalSectorSize.SelectedItem = ($State.Controls.cmbLogicalSectorSize.Items | Where-Object { $_.Content -eq $State.Defaults.generalDefaults.LogicalSectorSize.ToString() })
    
     # Populate Windows Release, Version, and SKU comboboxes
diff --git a/FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1 b/FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1
index 5f41546..94178cf 100644
--- a/FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1
+++ b/FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1
@@ -159,6 +159,9 @@ function Get-GeneralDefaults {
         Processors                     = 4
         VMLocation                     = $vmLocationPath
         VMNamePrefix                   = "_FFU"
+        SystemPartitionDriveLetter     = 'S'
+        WindowsPartitionDriveLetter    = 'W'
+        RecoveryPartitionDriveLetter   = 'R'
         LogicalSectorSize              = 512
         # Updates Tab Defaults
         UpdateLatestCU                 = $true
diff --git a/FFUDevelopment/config/Sample_default.json b/FFUDevelopment/config/Sample_default.json
index 49192e40e14e1bea8cc293d813b00522e6187fa1..a922b2c2b38e04b8558bc56a044eea9195676cb3 100644
GIT binary patch
delta 206
zcmZoN|6#FVj)-kCLq0<pLn=cNLnT81Ln4qaVaNpIe1<#*7a%_qEbjwUQv!q_l}Zd&
y3<^LpXmTU7@Z>WhJd>A*OtK9InotZ@kxQ~k!61`1n~CmWH6+y$;gc^)Z3F;1&@J)+

delta 24
gcmexi(PX}1j>zN>BC|H%65Yc(c>xFC<ZseV0F*TgO#lD@

diff --git a/docs/appsscriptvariables.md b/docs/appsscriptvariables.md
index 79be4c1..2336c7c 100644
--- a/docs/appsscriptvariables.md
+++ b/docs/appsscriptvariables.md
@@ -96,9 +96,11 @@ s{
   "Processors": 4,
   "ProductKey": "",
   "PromptExternalHardDiskMedia": true,
+  "RecoveryPartitionDriveLetter": "R",
   "RemoveApps": false,
   "RemoveFFU": false,
   "RemoveUpdates": false,
+  "SystemPartitionDriveLetter": "S",
   "Threads": 5,
   "UpdateADK": true,
   "UpdateEdge": true,
@@ -116,6 +118,7 @@ s{
   "VMLocation": "C:\\FFUDevelopment\\VM",
   "VMSwitchName": "External",
   "WindowsArch": "x64",
+  "WindowsPartitionDriveLetter": "W",
   "WindowsLang": "en-us",
   "WindowsRelease": 11,
   "WindowsSKU": "Pro",
diff --git a/docs/hyperv_settings.md b/docs/hyperv_settings.md
index bdc8b90..8ec6cb5 100644
--- a/docs/hyperv_settings.md
+++ b/docs/hyperv_settings.md
@@ -43,6 +43,20 @@ Default is `$FFUDevelopmentPath\VM`. This is the location of the VHDX that gets
 
 Prefix for the generated VM. Default is _FFU.
 
+## System Partition Drive Letter
+
+Drive letter used for the System partition while building the FFU VHDX. Default is `S`.
+
+## Windows Partition Drive Letter
+
+Drive letter used for the Windows partition while building the FFU VHDX. Default is `W`.
+
+## Recovery Partition Drive Letter
+
+Drive letter used for the Recovery partition while building the FFU VHDX. Default is `R`.
+
+These settings only affect FFU creation. They do not change the hard-coded drive letters used by `ApplyFFU.ps1` during deployment.
+
 ## Logical Sector Size
 
 Uint32 value of 512 or 4096. Useful for 4Kn drives or devices shipping with UFS drives. Default is 512.
diff --git a/docs/parameters_reference.md b/docs/parameters_reference.md
index 4ab0eed..e35dcc0 100644
--- a/docs/parameters_reference.md
+++ b/docs/parameters_reference.md
@@ -75,10 +75,12 @@ This table lists all top-level parameters in BuildFFUVM.ps1.
 | -Processors | int | Processors | Number of virtual processors for the virtual machine. Recommended to use at least 4. |
 | -ProductKey | string | Product Key | Product key for the Windows edition specified in WindowsSKU. This will overwrite whatever SKU is entered for WindowsSKU. Recommended to use if you want to use a MAK or KMS key to activate Enterprise or Education. If using VL media instead of consumer media, you'll want to enter a MAK or KMS key here. |
 | -PromptExternalHardDiskMedia | bool | Prompt for External Hard Disk Media | When set to $true, will prompt the user to confirm the use of media identified as External Hard Disk media via WMI class Win32_DiskDrive. Default is $true. |
+| -RecoveryPartitionDriveLetter | string | Recovery Partition Drive Letter | Drive letter used for the Recovery partition while building the FFU VHDX. Default is R. |
 | -RemoveApps | bool | Remove Apps Folder Content | When set to $true, will remove the application content in the Apps folder after the FFU has been captured. Default is $true. |
 | -RemoveDownloadedESD | bool | Remove Downloaded ESD file(s) | When set to $true, downloaded Windows ESD files are automatically deleted after they have been applied. Default is $true. |
 | -RemoveFFU | bool | Remove FFU | When set to $true, will remove the FFU file from the $FFUDevelopmentPath\FFU folder after it has been copied to the USB drive. Default is $false. |
 | -RemoveUpdates | bool | Remove Downloaded Update Files | When set to $true, will remove the downloaded CU, MSRT, Defender, Edge, OneDrive, and .NET files downloaded. Default is $true. |
+| -SystemPartitionDriveLetter | string | System Partition Drive Letter | Drive letter used for the System partition while building the FFU VHDX. Default is S. |
 | -Threads | int | Threads | Controls the throttle applied to parallel tasks inside the script. Default is 5, matching the UI Threads field, and applies to driver downloads invoked through Invoke-ParallelProcessing. |
 | -UnattendArm64FilePath | string | arm64 Unattend File Path | Path to the arm64 unattend XML source file used by Copy Unattend.xml and Inject Unattend.xml. Default is $FFUDevelopmentPath\Unattend\unattend_arm64.xml. |
 | -UnattendX64FilePath | string | x64 Unattend File Path | Path to the x64 unattend XML source file used by Copy Unattend.xml and Inject Unattend.xml. Default is $FFUDevelopmentPath\Unattend\unattend_x64.xml. |
@@ -98,6 +100,7 @@ This table lists all top-level parameters in BuildFFUVM.ps1.
 | -VMLocation | string | VM Location | Default is $FFUDevelopmentPath\VM. This is the location of the VHDX that gets created where Windows will be installed to. |
 | -VMSwitchName | string | VM Switch Name + Custom VM Switch Name (when Other selected) | Name of the Hyper-V virtual switch used when -EnableVMNetworking is set to $true. |
 | -WindowsArch | string | Windows Architecture | String value of 'x86', 'x64', or 'arm64'. This is used to identify which architecture of Windows to download. Default is 'x64'. |
+| -WindowsPartitionDriveLetter | string | Windows Partition Drive Letter | Drive letter used for the Windows partition while building the FFU VHDX. Default is W. |
 | -WindowsLang | string | Windows Language | String value in language-region format (e.g., 'en-us'). This is used to identify which language of media to download. Default is 'en-us'. |
 | -WindowsRelease | int | Windows Release | Integer value of 10, 11, 2016, 2019, 2021, 2022, 2024, or 2025. This is used to identify which Windows client/LTSC/server release to use. Default is 11. |
 | -WindowsSKU | string | Windows SKU | Edition/SKU to install. Accepted values are: 'Home', 'Home N', 'Home Single Language', 'Education', 'Education N', 'Pro', 'Pro N', 'Pro Education', 'Pro Education N', 'Pro for Workstations', 'Pro N for Workstations', 'Enterprise', 'Enterprise N', 'Enterprise 2016 LTSB', 'Enterprise N 2016 LTSB', 'Enterprise LTSC', 'Enterprise N LTSC', 'IoT Enterprise LTSC', 'IoT Enterprise N LTSC', 'Standard', 'Standard (Desktop Experience)', 'Datacenter', 'Datacenter (Desktop Experience)'. |