With any way to handle information, mistakes crop up. Just play "telephone" (whispering a message from person to person) with any group, and you'll see that the message that reaches the last person is wildly different from the message that started out. So it's really no surprise when backups crash.
What is surprising in this case, however, is that thus far, there seems to be no way of tracking down why it crashed, and how to repair the damage. Spoiler alert: in the end, I simply decided that the backups going back a little over two months were not valuable enough to keep trying, and I wiped the image they were on, and started a new backup from scratch. This does not mean the problem is solved, just that it was circumvented with losses I decided were acceptable.
How it happened: Well, I was using my 10.6.1 system as normal. I was running Growl 1.2b4, which I had heard from some quarters could interact weirdly with Time Machine (old versions of Growl, with old versions of Time Machine). At some point, I'm sure I shut down the computer in the middle of an over-the-wireless-network backup (to a .sparseimage, not on a Time Capsule, but a third-party drive connected to an Airport Extreme base station), which has never created a problem before.
But somewhere after some such shutdown in the middle of a backup (is this the cause?), the .sparseimage simply would not mount for Time Machine to backup, failing and "delaying" that backup attempt.
So I tried to open the .sparseimage in Disk Utility. It hung for quite a bit, but finally told me I couldn't (unfortunately, I didn't have the presence of mind to copy down the error message). So I disconnected the drive from the Airport Extreme and connected it directly via Firewire 800. This time, the icon changed to the same as a different user's folder, which you don't have access to (a folder icon with a red/white "no access"-style circular symbol over it). Attempting to do anything with it just resulted in being told that I didn't have privileges to access it, but oddly didn't provide any place at all where I could have entered an admin password (possibly because the only password with access to it would have been the root password, and the OS is constructed such that the only way to enter that is via the Install DVD, if the computer has a root password set at all. It's big juju to mess with anything that requires a root password!).
So I gave up, and trashed the apparently corrupted .sparseimage, then created a completely new Time Machine backup for the drive. The only way in which it seems to function differently (other than only having backups starting from the new date) is that the backup name is now "Time Machine Backups", rather than the "Backup of [My computer's name]" as it had been, since Leopard.
I was sad to see the old backups go, but anything that I've needed to restore from backup (to my knowledge) has already been restored. An added bonus is that I reclaimed quite a lot of disk space. Still, it would have been nice never to have had to trash the old backup, and restart a new one. Does anyone out there have similar experiences, and any suggestions I might not have tried?
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