Convert-WindowsImage.ps1 Alternative

[jontywigster]

Hi,

I've no idea if this of interest at all, or whether it was already discussed. Apologies!

I built a small set of scripts to build Linux and Windows Hyper-V VMs too, but only in a way suitable for my environment. I originally built Windows VMs from WDS but this takes ages so I hunted for ways to speed up the install, and found this fantastic repo. Congrats!

I was curious about Convert-WindowsImage.ps1 but it seems to have some strict requirements about the PowerShell environment it can run in. I fiddle in VS Code and couldn't figure out how to use it but I don't really know what I'm doing in PoSh.

I took at look at what Convert-WindowsImage.ps1 does, couldn't figure out various bits, but came up with some terrible hackery -

# Mount the source ISO
Mount-DiskImage -ImagePath $sourceISO
$mountedISO = Get-DiskImage -ImagePath $sourceISO | Get-Volume
$mountedDriveLetter = $mountedISO.DriveLetter
if (-not $mountedDriveLetter) {
    Dismount-VHD -Path $outputVHDX
    throw "Failed to get the drive letter of the mounted ISO."
}

$mountedDriveLetterPath = $mountedDriveLetter + ":\"

# Create the VHDX
$SizeBytes = "60GB"
New-VHD -Path $outputVHDX -Dynamic -SizeBytes $SizeBytes
Mount-VHD -Path $outputVHDX
$vhd = Get-Disk | Where-Object { $_.Location -eq "$outputVHDX" }

# Initialize, partition, and format the VHDX
Initialize-Disk -Number $vhd.Number -PartitionStyle GPT

# Create the EFI System Partition
$systemPartition = New-Partition -DiskNumber $vhd.Number -Size 100MB -GptType "{c12a7328-f81f-11d2-ba4b-00a0c93ec93b}" -AssignDriveLetter
Format-Volume -Partition $systemPartition -FileSystem FAT32 -NewFileSystemLabel "System"

# Create the MSR Partition
New-Partition -DiskNumber $vhd.Number -Size 16MB -GptType "{e3c9e316-0b5c-4db8-817d-f92df00215ae}"

# Create the Windows Partition
$windowsPartition = New-Partition -DiskNumber $vhd.Number -UseMaximumSize -GptType "{ebd0a0a2-b9e5-4433-87c0-68b6b72699c7}" -AssignDriveLetter
Format-Volume -Partition $windowsPartition -FileSystem NTFS -NewFileSystemLabel "Windows"

# Create directory structure on EFI System Partition
New-Item -Path "$($systemPartition.DriveLetter):\EFI" -ItemType Directory -Force

$mapping = @{
    "25s"  = "*Standard*Desktop*"
    "25d"  = "*Datacenter*Desktop*"
    "25sc" = "* Standard *"
    "25dc" = "* Datacenter *"
}
  
$installWimPath = $mountedDriveLetterPath + "sources\install.wim"
$imageInfo = Get-WindowsImage -ImagePath $installWimPath
$imageInfo = $imageInfo | Sort-Object ImageName

#lookup image index of $os using mapping
$imageIndex = $null
$pattern = $mapping[$os]
foreach ($image in $imageInfo) {
    if ($image.ImageName -like $pattern) {
        $imageIndex = $image.ImageIndex
        $imageName=$image.ImageName
        break
    }
}

if ($null -eq $imageIndex) {
    Dismount-VHD -Path $outputVHDX
    Dismount-DiskImage -ImagePath $sourceISO
    Remove-Item -Path $outputVHDX -Force
    throw "No matching image found for $os."
}

Write-Host "dism apply, will be slow - $imageName"
dism /Apply-Image /ImageFile:${mountedDriveLetterPath}sources\install.wim /Index:$($image.ImageIndex) /ApplyDir:$($windowsPartition.DriveLetter):\ /CheckIntegrity

#prep boot files
bcdboot "$($windowsPartition.DriveLetter):\Windows" /s "$($systemPartition.DriveLetter):" /f ALL

# Copy unattend file to Windows partition
Copy-Item -Path $UnattendPath -Destination "$($windowsPartition.DriveLetter):\Windows\Panther\Unattend.xml" -Force

# Dismount the VHDX and the ISO
Dismount-VHD -Path $outputVHDX
Dismount-DiskImage -ImagePath $sourceISO

My only goal was to be able to create a VHDX from an ISO, in VS Code. Obviously, my bodging doesn't do everything Convert-WindowsImage.ps1 does but I can't even figure out what most of that large script does. I really don't know what I'm doing in PoSh so might have done this for nothing.

Again, I have no idea if useful in any way. It's just a start of some ideas, has terrible error handling and other bits so far, and is not meant with any disrespect to the convert script, or this repo.

Cheers.

[fdcastel]

Nice work, @jontywigster!

Convert-WindowsImage.ps1 is an ancient script that, to the best of my knowledge, was originally developed by a former Microsoft employee but has not been maintained.

As a result, it is only compatible with Windows PowerShell 5.1 and does not support newer versions such as PowerShell Core (6.0).

Additionally, the script was initially designed with a graphical user interface, adding to its complexity.

That said, its core functionality remains similar to your script: it generates a VHDx with the necessary partitions and applies an image using DISM.

I never prioritized porting it to PowerShell Core because (until now) all my use cases allowed me to simply use powershell instead of pwsh. I'm not sure why you specifically need to use the latter in VSCode, but I’m sure you have your reasons.

Keep up the great work!

P.S.: As a suggestion, maybe you could create a gist? It would be a more convenient way to track and manage the evolving code.

[jontywigster]

Hello!

A GUI! I started skipping over large chunks I thought weren't relevant (to me) so never thought of this. Thanks.

I'm not sure why you specifically need to use the latter in VSCode, but I’m sure you have your reasons.

It just seemed like the hot new thing, and I understood 5.1 wasn't getting new features. And I'm too lazy to create a new profile in VS Code...

P.S.: As a suggestion, maybe you could create a gist?

https://gist.github.com/jontywigster/a12d40457d4886631925fa5182a43868

Thanks again!

[fdcastel]

5.1 wasn't getting new features.

Correct. However, it is still supported. And given that Windows Server 2025 includes it, you can reasonably expect support to continue for at least another ten years.

https://learn.microsoft.com/en-us/powershell/scripting/install/powershell-support-lifecycle

I still have A LOT of build an deploy scripts in Windows Powershell (5.1).

And I'm too lazy to create a new profile in VS Code...

But not quite lazy enough to port the entire functionality of a battle-tested script to another platform! 😄👍🏻

[JamesMilne]

just chipping in my two cents, I was looking at this as well. For me, I had a very large number of VMs to script up and I noticed how much time this spent "Converting ISOs to VHDx" files, so I started to implement a caching mechanism.. to only create the vhdx once, and to store it in a "cache" folder. Then subsequent calls could be a copy operation rather than a convert operation..

[jontywigster]

Hi,

But not quite lazy enough to port the entire functionality of a battle-tested script to another platform!

I didn't realise 5.1's going to be supported for so long. Nice! My stuff is only for me so I'm not worried about battle-hardening.

I've disturbed you much more than I intended, thanks again.

Should I close this issue? I see JamesMilne has commented about caching so perhaps it should stay open.

Anyway, cheers!

[fdcastel]

No problem, @jontywigster.

Issues actually are a great way to track problems effectively.

So, I have enabled GitHub Discussions and migrated your thread there. We can continue the talk here.

First post! 😄🚀