Changes made to how drivers are applied to the FFU and WinPE Deployment media

[rbalsleyMSFT]

What Changed?

I just pushed a commit that changes how drivers are applied to the FFU and WinPE deployment media. In order to handle long driver paths (both long model names and deeply nested folders from driver manufacturers), I've opted to use SUBST to shorten the driver path. If you don't know what SUBST is, it simply associates a path with a drive letter. Think mapped network drive, but local.

I've also made a change to log the console output of dism when applying drivers (this goes back to using the Invoke-Process function and calling native dism instead of add-windowsdriver.)

Why Make This Change?

The reason I've moved to this approach is due to dism's inability to support long paths which is 260 characters. Some apps and tools can leverage putting a \\?\ in front of a path and that will allow the OS to resolve a long path. While dism can read a path with \\?\ in it, if the path is too long, it will fail to process the path. FFUBuilder uses this trick in some places.

The lack of long path support causes issues where dism will fail to add drivers to FFU/WIM files. In situations where it appears that dism has applied the drivers you selected, you might find that if you check device manager, there might be a number of things missing. You might also notice that WinPE isn't detecting your hard drive even though you expected the drivers to be there. Drivers with long paths would just silently fail to apply.

Example Logging

Here's what you'll now see in the FFUDevelopment.log

1/9/2026 12:21:01 PM Scanning for INF files under: C:\FFUDevelopment\Drivers
1/9/2026 12:21:01 PM Driver injection will process 22 SUBST-safe folders (candidateFolders=39, INF total=143, substMaxLen=240).
1/9/2026 12:21:02 PM Using SUBST drive Z: for driver injection loop.
1/9/2026 12:21:02 PM [1/22] Mapping 'C:\FFUDevelopment\Drivers\Dell\Dell Pro Desktops Dell Pro Micro Plus QBM1250-Dell Pro Micro QCM1250,Dell Pro Micro Plus XE QBM1250 (0D05)\Audio\Realtek-High-Definition-Audio-Driver_88FN7_WIN64_6.0.9768.1_A00\0\Drivers\I' to Z: with SUBST.
1/9/2026 12:21:03 PM dism.exe /Image:"C:\FFUDevelopment\Mount" /Add-Driver /Driver:Z:\ /Recurse
1/9/2026 12:21:11 PM 
Deployment Image Servicing and Management tool
Version: 10.0.26100.5074

Image Version: 10.0.26200.7462

Searching for driver packages to install...
Found 14 driver package(s) to install.
Installing 1 of 14 - Z:\DetectionVerificationDrv.inf: The driver package was successfully installed.
Installing 2 of 14 - Z:\gna.inf: The driver package was successfully installed.
Installing 3 of 14 - Z:\IntcAudioBus.inf: The driver package was successfully installed.
Installing 4 of 14 - Z:\IntcBTAu.inf: The driver package was successfully installed.
Installing 5 of 14 - Z:\IntcBtLE.inf: The driver package was successfully installed.
Installing 6 of 14 - Z:\IntcDMic.inf: The driver package was successfully installed.
Installing 7 of 14 - Z:\IntcOED.inf: The driver package was successfully installed.
Installing 8 of 14 - Z:\IntcSDW.inf: The driver package was successfully installed.
Installing 9 of 14 - Z:\IntcSdwBus.inf: The driver package was successfully installed.
Installing 10 of 14 - Z:\IntcSST.inf: The driver package was successfully installed.
Installing 11 of 14 - Z:\IntcStreaming.inf: The driver package was successfully installed.
Installing 12 of 14 - Z:\IntcUSB.inf: The driver package was successfully installed.
Installing 13 of 14 - Z:\IntelMvaExtensionInf\IntelMvaExtension.inf: The driver package was successfully installed.
Installing 14 of 14 - Z:\LT6911Au.inf: The driver package was successfully installed.
The operation completed successfully.

1/9/2026 12:21:11 PM Removing SUBST drive Z:

How This Works

Both dism and subst do not support long paths (and dism doesn't support recursing junctions), so a risk is that if you create a subst path that's too long you'll run into the same issue that you have with dism. So you have to make sure that the path you want to create with subst is shorter than 260 characters.

Based on looking at the above logging, you might wonder why not just create the SUBST path up to the model and just loop through each model if multiples exist (e.g. C:\FFUDevelopment\Drivers\Dell\Dell Pro Desktops Dell Pro Micro Plus QBM1250-Dell Pro Micro QCM1250,Dell Pro Micro Plus XE QBM1250 (0D05) maps to Z:\) and while initially that might make sense, depending on the driver manufacturer, even they can create extremely long paths (Liteon camera drivers for Lenovo do this). This should hopefully solve for both long model names and long driver folder names.

So the way this works is:

• Find the INFs and start at the INF parent folder
• If path length is longer than 240 characters, walk up the path until it's shorter than 240
• Do some deduplication work to make sure to avoid any redundant folders when the parent already covers them

FYI

This will make it so adding drivers to the FFU or WinPE media will take a little longer since we have to run subst and dism many times (1 min vs 2 min or so). But in testing it's been much more reliable. There's also better logging now so you can see what was added or what failed.

I also contemplated normalizing model names, but that would end up in a game of whack a mole any time an OEM creates another model. The intent here is to have a process that would be OEM/model agnostic. I also tried junctions but dism fails to recurse through junctions, however it will work if you target a specific INF file.

So in the end, this seems to work the best in my testing so far. If you see anything strange, let me know, but this should hopefully fix some of the random issues where some drivers may not be applying as expected.