no mountable file systems - error opening sparseimage file

[VirtualWolf]

I've now booted successfully from the restored backup image on my spare partition. Now interestingly, when I went to mount the sparsebundle in the Finder in preparation for copying it back onto the internal drive of the Mac mini, I got the same "No mountable filesystems" error! I opened Disk Utility, dragged the sparsebundle into it, then selected it and hit Open in the toolbar, and it still didn't mount.

Even though the permissions on the whole sparsebundle is owned by the admin user and the regular user I run in isn't one, they're set to rwxr-xr-x, so I ought to be able to read the sparsebundle properly anyway. I logged into that admin account, and can mount the sparsebundle perfectly.

I had made a copy of it from my regular non-admin user on another partition, and in that same non-admin account I can mount /that/ copy fine! So it would appear to be some bizarre permissions-related thing that was preventing me from mounting it.