First, make sure it's running, and then disconnect from the drive it's backing up to. Yes, that's obviously a dumb thing to do, but let me explain how I got there :)

I started using a drive attached to one of my other Macs across my network recently, That mostly works pretty well - but the downside is this: nearly every night I pull my MBP off the wire, get on my wireless LAN, and take it into the living room. Two days ago, I did that without noticing that Time Machine was running. Had it failed cleanly, that wouldn't have been a problem - but when you do network backups, they get stored into what the Mac calls a "sparse bundle" file. Time Machine mounts that as a directory, but if you open a terminal, it just looks like a big file.

When I disconnected, there was an "in progress" file (kind of like a lock file, I guess) lurking in the bundle. That file didn't get deleted, so every time Time Machine fired up after that (hourly), it would see the "in progress" backup and fail.

If the lock file were a simple file or directory, the solution would be simple - but since it's only visible as a file after it's mounted, I just waited for Time Machine to start, navigated to the mounted file system, and blew the lock file (directlry, really) away. I probably should have used Disk Utility to mount the file, but I wasn't really thinking about that. Now Time Machine works again - and I'll be much more careful about unhooking from the wired LAN in the future...

Technorati Tags: Time Machine, backups, NAS