From 44aa4d3a324fff522460bdd7d229127895d0b203 Mon Sep 17 00:00:00 2001
From: rbalsleyMSFT <53497092+rbalsleyMSFT@users.noreply.github.com>
Date: Wed, 19 Nov 2025 23:08:39 -0800
Subject: [PATCH] Add SUBST drive mapping for driver injection to handle long
 paths

Introduces helper functions to create and remove SUBST drive mappings for driver folders, addressing potential issues with long file paths during driver injection.

The new implementation:
- Adds `Get-AvailableDriveLetter` to find an unused drive letter
- Adds `New-DriverSubstMapping` to create a virtual drive mapping to the source folder
- Adds `Remove-DriverSubstMapping` to clean up the mapping after use
- Updates the driver injection workflow to use the mapped drive path instead of the original path
- Wraps the operation in try-catch-finally to ensure cleanup occurs even on errors

This approach mitigates issues with excessively long driver folder paths that could exceed command-line limits or cause path-related failures during DISM operations.
---
 .../WinPEDeployFFUFiles/ApplyFFU.ps1          | 91 ++++++++++++++++---
 1 file changed, 80 insertions(+), 11 deletions(-)

diff --git a/FFUDevelopment/WinPEDeployFFUFiles/ApplyFFU.ps1 b/FFUDevelopment/WinPEDeployFFUFiles/ApplyFFU.ps1
index d7e5d12..5da5222 100644
--- a/FFUDevelopment/WinPEDeployFFUFiles/ApplyFFU.ps1
+++ b/FFUDevelopment/WinPEDeployFFUFiles/ApplyFFU.ps1
@@ -738,13 +738,67 @@ function Test-DriverFolderHasInstallableContent {
     
         return $false
     }
-    catch {
-        WriteLog "Failed to inspect driver folder '$Path': $($_.Exception.Message)"
-        return $false
+        catch {
+            WriteLog "Failed to inspect driver folder '$Path': $($_.Exception.Message)"
+            return $false
+        }
     }
-}
     
-#Get USB Drive and create log file
+    function Get-AvailableDriveLetter {
+        $usedLetters = (Get-PSDrive -PSProvider FileSystem).Name | ForEach-Object { $_.ToUpperInvariant() }
+        for ($ascii = [int][char]'Z'; $ascii -ge [int][char]'A'; $ascii--) {
+            $candidate = [char]$ascii
+            if ($usedLetters -notcontains $candidate) {
+                return $candidate
+            }
+        }
+        return $null
+    }
+    
+    function New-DriverSubstMapping {
+        [CmdletBinding()]
+        param(
+            [Parameter(Mandatory = $true)]
+            [string]$SourcePath
+        )
+    
+        $resolvedPath = (Resolve-Path -Path $SourcePath -ErrorAction Stop).Path
+        $driveLetter = Get-AvailableDriveLetter
+        if ($null -eq $driveLetter) {
+            throw 'No drive letters are available for SUBST mapping.'
+        }
+        $driveName = "$driveLetter`:"
+        $mappedPath = "$driveLetter`:\"
+        WriteLog "Mapping driver folder '$resolvedPath' to $driveName with SUBST."
+        $escapedPath = $resolvedPath -replace '"', '""'
+        $arguments = "/c subst $driveName `"$escapedPath`""
+        Invoke-Process -FilePath cmd.exe -ArgumentList $arguments
+        return [PSCustomObject]@{
+            DriveLetter = $driveLetter
+            DriveName   = $driveName
+            DrivePath   = $mappedPath
+        }
+    }
+    
+    function Remove-DriverSubstMapping {
+        [CmdletBinding()]
+        param(
+            [Parameter(Mandatory = $true)]
+            [string]$DriveLetter
+        )
+    
+        $driveName = "$DriveLetter`:"
+        WriteLog "Removing SUBST drive $driveName"
+        try {
+            $arguments = "/c subst $driveName /d"
+            Invoke-Process -FilePath cmd.exe -ArgumentList $arguments
+        }
+        catch {
+            WriteLog "Failed to remove SUBST drive $($driveName): $_"
+        }
+    }
+        
+    #Get USB Drive and create log file
 $LogFileName = 'ScriptLog.txt'
 $USBDrive = Get-USBDrive
 New-item -Path $USBDrive -Name $LogFileName -ItemType "file" -Force | Out-Null
@@ -1481,12 +1535,27 @@ if ($null -ne $DriverSourcePath) {
         }
     }
     elseif ($DriverSourceType -eq 'Folder') {
-        WriteLog "Injecting drivers from folder: $DriverSourcePath"
-        Write-Host "Injecting drivers from folder: $DriverSourcePath"
-        Write-Host "This may take a while, please be patient."
-        Invoke-Process dism.exe "/image:W:\ /Add-Driver /Driver:""$DriverSourcePath"" /Recurse"
-        WriteLog "Driver injection from folder succeeded."
-        Write-Host "Driver injection from folder succeeded."
+        $substMapping = $null
+        try {
+            $substMapping = New-DriverSubstMapping -SourcePath $DriverSourcePath
+            $shortDriverPath = $substMapping.DrivePath
+            WriteLog "Injecting drivers from folder via SUBST. Source: $DriverSourcePath, Mapped: $($substMapping.DriveName)"
+            Write-Host "Injecting drivers from folder: $shortDriverPath"
+            Write-Host "This may take a while, please be patient."
+            Invoke-Process dism.exe "/image:W:\ /Add-Driver /Driver:$shortDriverPath /Recurse"
+            WriteLog "Driver injection from folder succeeded."
+            Write-Host "Driver injection from folder succeeded."
+        }
+        catch {
+            WriteLog "An error occurred during folder driver installation: $_"
+            Invoke-Process xcopy.exe "X:\Windows\logs\dism\dism.log $USBDrive /Y"
+            throw $_
+        }
+        finally {
+            if ($null -ne $substMapping) {
+                Remove-DriverSubstMapping -DriveLetter $substMapping.DriveLetter
+            }
+        }
     }
 }
 else {