If you're using Time Machine to back up Macs to a Time Capsule or networked hard drive, each backup is contained in a single sparse disk image bundle (.sparsebundle) disk file. When your backup contains weeks' or months' of data, if that single file is lost or damaged, you'll obviously lose all of your backups. It is therefore quite important to back up the sparse bundle and test the backup! Here's one way to do that that, though there are many other methods available:
- Copy the sparse bundle back to your Mac (and then remember to exclude it from Time Machine's backups, using the Options button in the Time Machine System Preferences panel).
- Use Disk Utility's Repair Disk feature on the mounted .sparesebundle to insure that it's in good shape. This should be quite fast when run on the local copy of the sparse image bundle.
[robg adds: I've had my MacBook Pro's Time Machine backup go bad a couple of times now, so I've taken to copying the sparse image file onto the Mac Pro as a redundant backup. I keep it in a folder that has been added to Time Machine's 'do not backup' list, so that it's not taking a ton of space in my Mac Pro's own Time Machine backups. If you have only one Mac, I suggest attaching a FireWire hard drive and using that to store the copy of your sparse image bundle, as drive space on a laptop is a precious commodity.]

