This post documents how to hack a Windows registry entry to gain access to the contents of a WSL2 .vhdx volume which resides within a Windows 10 System Image on an external disk.

Scenario

Everyone should have a backup strategy that works for them in terms of hassle factor and capability: your strategy should have no more hassle than you are willing to put up with to give you a daily backup capability.

On macOS, I use the excellent Carbon Copy Cloner. On Windows 10 I use the built-in Create a system image. In both cases, I store the backup on an external drive. Yesterday, due to a stupid git rebase user error, I lost some commits on a repo that was stored within my WSL2 Ubuntu machine. “No worries,” I thought. “I’ll just grab the commits from my backup”. I followed these steps to mount the System Image as a local drive and navigate to the file…“oh wait. The file is within the WSL image. [:sad trombone:]”.

With help from WSL2 Twitter celebrity Craig Loewen I used this process to get my lost commits.

If you don’t already have WSL2, you should totally get it. Get started at <https://aka.ms/install-wsl>. I was a Mac user from 2007 until I joined Microsoft in June 2019. If not for WLS2, I’d still be a Mac user.

How to Access WSL2 .vhdx on External Drive Within a Windows 10 System Image

We will edit the Windows registry to point a new Debian WSL2 distribution to the .vhdx file on the external drive.

Get the fully qualified path to the .vhdx file

Mount the System Image that contains the .vhdx file and capture its fully qualified path. In my case this is:

K:\Users\edburns\AppData\Local\Packages\CanonicalGroupLimited.UbuntuonWindows_79rhkp1fndgsc\LocalState

Create a fresh WSL2 Debian distribution so you can edit its Windows Registry entry without fear of damaging anything

  1. Make it so when you do wsl -l -v in Windows PowerShell you do not see a distribution named Debian. If you do see one, figure out how to save it aside. We want to install a fresh Debian distribution. If you already have one, you’ll need to move it aside so you don’t damage it.

  2. Install a fresh Debian (RIP Ian Murdock) distribution from the Windows store. Create a userid when asked.

  3. Exit the Debian and shut down WSL2 with the command wsl --shutdown in PowerShell.

Edit the Windows Registry entry for the .vhdx file of the fresh Debian distribution

This is the secret sauce! I use regedt32, but you can use any other registry editor. In your registry editor, navigate to this entry and expand it.

Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Lxss

This image shows what I see under the Lxss entry.

Entries within Lxss

Carefully select each entry until you find the one whose DistributionName sub-entry is exactly Debian.

Select the BasePath sub-entry and Modify its value. Set the value to be exactly the fully qualified path to your external .vhdx file, not including the filename or trailing slash.

Exit the Windows Registry.

Run the “Debian” image and get your files

In Windows PowerShell, run wsl -d Debian. This should boot up the WSL2 into the .vhdx, allowing you to get the missing files.

PRO TIP!

The super handy \\wsl$\<distroname> usage in Windows File Explorer should work also. In my case I was able to get the files by visiting \\wsl$\Debian\home\edburns\workareas in Windows Explorer.

Thanks again to Craig Loewen for this tip.