Backing up a partition to disk image unmounts that partition

[dnanian]

OK. This really sounds like a bug in AFP, which is not properly dealing with the unmount of the sparse image.

As I mentioned before, the User's Guide describes what's going on during the 'direct imaging' process. Multiple steps are required to ensure the backup is of minimal size, and to allow it to be restored, using ASR, to any device (it's device-independent). Instead, though, I'd strongly recommend using the technique in the FAQ, as I suggested before. I think this will get you past this strangeness while we try to determine what, exactly, is happening. (It really sounds like AFP is erroneously marking the volume as unavailable even though it's not unmounted.)