How to add drivers to a WinPE Flash Drive

First create a folder to temporarily mount the existing image.  Then create a folder to unzip the drivers too. Take note of the full path of your temporary folder and your driver folder. In this example I am adding the Intel VMD/RST driver.

#Mount the WinPE image from the flash drive
Dism /Mount-Image /ImageFile:"D:\sources\boot.wim" /index:1 /MountDir:"C:\Users\testaccount\TempMount"

#Use the below command to see what drivers are currently added to the WinPE image
Dism /Image:C:\Users\testaccount\TempMount /Get-Drivers

#Add the driver you unzipped to the driver folder.
Dism /Add-Driver /Image:"C:\Users\testaccount\TempMount" /Driver:"C:\Users\testaccount\15063\Drivers\VMD\iaStorVD.inf"

#Run the below to unmount and commite the modified image back
Dism /Unmount-Image /MountDir:"C:\Users\testaccount\TempMount" /Commit

#You might have to run this if having trouble unmounting the image
dism /cleanup-wim