Unmountable Sparse Image

[diskinserted]

It didn't go down by 16 GB. It went from 60GB to 44 MB! The G5 was showing it as a 60 GB file until the minute I tried to move it over to the mini, at which point it gave me the unmountable message. Immediately after that, I got info on the dmg file, and it had it as 44 MB. I just don't see how the HD could suddenly see a drop of 59.9 GB. Should I assume that the file was corrupt from the get-go? I just don't see how it could be taking up so much real estate, then go to such a small size, almost instantly. I will try the DU repair and see what happens.

Is there any way, for future reference, that I can periodically check on a dmg file and make sure that everything is cool? If this is indeed lost, this is a BIG loss. I'd like to try to avoid it in the future. Would running something like Data Rescue work? I've avoided restarting the machine in the hopes that something could be salvaged. Why would it appear to have made that back-up only to have created a file bloated with nothing? Is there anything that can be done? DU showed

"Verify and Repair disk “disk2s2”
Checking HFS Plus volume.
Invalid B-tree node size
Volume check failed."