A customer recently had a requirement for rebuilds to be done in remote sites via USB flash drives configured as MCM Bootable Media due to a lack of local MCM Distribution Points and PXE Boot capability.

Using devices in UEFI mode with BitLocker enabled makes this tricky when the Boot Image associated with the Task Sequence becomes out of sync with the Boot Image on the USB media. If the boot images don’t match then MCM attempts to pre-stage onto the local disk and fails as the OSDisk is unavailable due to it being encrypted with BitLocker (the drive appears as “RAW” and cannot be accessed) and none of the other partitions are large enough or available.

I worked around this by creating a PowerShell PreStart script and adding it to the Boot Media ISO image. The script runs before the Task Sequence begins. It creates a Diskpart configuration text file on the fly in the ‘X:\Windows\Temp’ folder of the running WinPE. After creating the Diskpart configuration file, it then runs Diskpart referencing the configuration file in order to create suitably-sized/lettered partitions to successfully boot from using UEFI and that are also accessible for the Task Sequence to download and pre-stage the latest Boot Image if it’s required (i.e. if it’s different to the boot image on the USB).

Problem solved!

The command for the PreStart script that I used was:

cmd /C PowerShell.exe -ExecutionPolicy ByPass -File PreStart.ps1

And the PowerShell code contained with PreStart.ps1 is shown below:

<#
.DESCRIPTION
    Configures GPT disk layout using DiskPart.exe to avoid Boot Image mismatching when using MCM Bootable Media
.EXAMPLE
    PowerShell.exe -ExecutionPolicy ByPass -File .ps1
.NOTES
    Author:         Jonathan Conway
    Modified:       06/04/2019
    Version:        1.0
#>

# Display warning and request confirmation from engineer
$Shell = New-Object -ComObject "WScript.Shell"
$Button = $Shell.Popup("Proceeding will wipe all local data from all local drives. Hold Power Button until device powers off to cancel. Click OK to proceed.", 0, "WARNING", 0)

# Set variables
$DiskPartFile = "X:\Windows\Temp\DiskpartConfig.txt"

if (Get-Volume | Where-Object {$_.DriveLetter -eq 'C' -and $_.DriveType -eq 'Removable'}) {
Get-Partition -DriveLetter 'C' | Set-Partition -NewDriveLetter 'U'
}

# Create contents of DiskPart configuration file
Write-Output "SELECT DISK 0" | Out-File -Encoding utf8 -FilePath "$DiskpartFile"
Write-Output "CLEAN" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "CONVERT GPT" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "CREATE PARTITION EFI SIZE=200" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "ASSIGN LETTER=S" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "FORMAT QUICK FS=FAT32" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "CREATE PARTITION MSR SIZE=128" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "CREATE PARTITION PRIMARY" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "ASSIGN LETTER=C" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "FORMAT QUICK FS=NTFS" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append
Write-Output "EXIT" | Out-File -Encoding utf8 -FilePath "$DiskpartFile" -Append

# Run DiskPart
Start-Process -FilePath "diskpart.exe" -ArgumentList "/s $DiskPartFile" -Wait

In my environment this formats the disks in a way which allows my Task Sequence to progress whatever state the UEFI partitions are in (i.e. BitLocker enabled or not).

A pop up warning is shown on screen stating:

“Proceeding will wipe all local data from all local drives. Hold Power Button until device powers off to cancel. Click OK to proceed“.

Clicking OK continues ahead and starts the Diskpart process before progressing to the Task Sequence selection screen 🙂

/ JC