[dnanian]

The unmounting volumes, though, indicate a hardware problem, as I said... and the error is coming from Disk Utility, which isn't something we wrote, and might have a bug in it.

If you have another Mac, I'd suggest restoring the (mounted) image with Smart Update, using the Mac, rather than Disk Utility. We have far better diagnostics than Disk Utility does, and you're far more likely to get a reasonable error message if there's a hardware (or other) problem going on here.

As far as "less than reliable" goes: we're all at the mercy of the OS, the hardware, your disk surface, etc. There are always things that can go wrong... with anything. That's why we provide support.