sparsebundle = "no mountable file system"

[Breeze]

I tried to do the weekly SuperDuper backup of my 2007 MBP Lion 10.7.5 boot drive but it failed with the message that the sparsebundle had "no mountable file system". The drive is a 500 GB Samsung in a FW-800 enclosure which has only been used for backups; it runs for a few hours every week or so. I'd be surprised if there's something mechanically wrong with the drive.

In Disk Utility, the drive itself comes up clean. If I run a verify on the sparsebundle backup image, it fails saying "this volume must be repaired".

Obvious questions:

1. is this backup simply borked? Should I trash it and start a new one?

2. If the drive is ok, what could have trashed the image file? There was an unusual amount of disk activity that lasted a good 30-45 seconds when I first connected the drive, which was much more than normal.

3. If I try to "repair" the image file with disk utility, will it in fact repair the image and allow me to continue using this image file for backups? Is it actually possible to repair a sparsebundle image file?

I've been using this drive almost exclusively for my SD backups for the last couple of years, and I've been able to mount the backup image many times.

Thanks for any help!