Yesterday my external Western Digital disk (which I use for Time Machine, among other things) stopped mounting automatically when I plug it in. This may *cough*definitely*cough*have something to do with me unmounting it forcefully while it was doing a backup, since I was in a hurry.
Disk Utility reported everything was OK with the disk, but it still failed to mount. Upon poking a bit deeper, I found these messages in the logs: