You make a full disk backup to encrypted dmg on external volume. You’re doing system upgrade using a clean installation, and then you try to bring your data back from the DMG you just did and you see a cheerful message:
“Unable to mount, no mountable filesystem”
You have checked before if the disk image will mount and everything was fine. Your first thought is…. bummer.
What you can do is to try to attach the volume from Terminal.app
hdiutil attach -ignorebadchecksums -stdinpass -nomount -noverify your_image.dmg
and then you can use some recovery software (e.g. like Data Rescue) to restore available files.