Philip Hendry believes he’s stumbled upon a rather disturbing bug in Time Machine:

I’ve been a long time fan of yours since your days at TechTV and am glad to see you answering questions again. I would like to inform you and the rest of the community to a bug I discovered in Time Machine on OS X “Leopard”. I have been using Leopard since it’s release (waited in line for 2 hours for one of the first copies) and have been relying on Time Machine for just as long.

Apparently sometime in the last few days, my Time Machine drive filled to capacity. Knowing that it would automatically begin deleting my old backups to free space, I was not concerned. However, this morning my backup failed to this error:

“Unable to complete backup. Unable to create backup directory.”

After exhausting my search of Apple’s support page and trying a few things. I resorted to calling Applecare for assistance. After about 45 minutes of troubleshooting and talking to other reps, my rep came back as dumbfounded as I was. I was told that instead of deleting older backups, as advertised, Time Machine instead begins compressing the older ones. If you run out of space before the compression can catch up, Time Machine will fail, as it does not have enough space to create a new backup directory. The only way to restore functionality is to completely erase your backup drive and start from scratch.

Hopefully this bug will be fixed in the next update to Leopard. Just thought I would share this with you and the rest of the community. Congratulations on your recent switch to the Mac. I hope that it goes as smoothly for you as it did for me (Got my first computer at 5 years old. Spent 14 years on Windows).

Or archive the drive and add another, which isn’t a full solution – but at least you won’t have to lose the archives you created already. Figured I’d throw this post out there for anybody else running into the problem (or for Windows users to understand that OS X isn’t perfect either).