@matlock: I ran out of replies on your post How You Can Help Test Ubuntu 20.04 LTS on WSL because I’m a new user, and couldn’t edit my replies, so I thought I post it here because it’s related.
As mentioned in my reply, wsl.exe --export Ubuntu ubuntu-backup.tar.gz won’t actually compress to a tar.gz,- you’d need to pipe to gzip wsl.exe --export Ubuntu -| gzip > ubuntu-backup.tar.gz. (notize the ‘-’ at the end)
But I had a lot of problems importing it directly through tar -xOzf ubuntu-backup.tar.gz| wsl.exe --import UbuntuFocal C:\UbuntuFocal ubuntu-backup.tar.gz - so I resorted to 7zip to do the export/import thing:
wsl.exe --export Ubuntu - | 7z a -si ubuntu-backup.tar.7z
7z x -so ubuntu-backup.tar.7z | wsl --import UbuntuFocal C:\UbuntuFocal -
The uncompressed tar of a ubuntu dist is quite large so this saves a bit of wear and tear on our precious ssd’s 
Sorry to hijack, but I thought it was relevant in our common quest to mitigate the u20.04/wsl1 bug.