When your disk drive starts making clicking noises as if it were a metronome... That's a sign.
When your disk drive starts throwing I/O Errors to the console... That's a sign.
When your disk drive no longer mounts its volume... That's a sign.
Yeah - A sign that your disk is about to die or is already dead.
That's what happened to my Time Machine disk that is attached to my Apple iMac desktop computer. It was a 750GB Maxtor One Touch III, purchased just 7 months ago.
The Good News is that it was just my backup drive and not my primary drive. My primary drive is a 1TB disk, as shipped by Apple, and thankfully has hummed along perfectly. So, I could keep working.
The Bad News is that you are no longer making hourly backups, and all of your Time Machine backups are no longer available, so you are completely exposed. I.e., I need a Time Machine backup disk for my Time Machine disk! Or, multiple Time Machine volumes (if that's possible?)...
So, I surf over to the Seagate/Maxtor site and check the warranty coverage. The disk has a 1-year warranty, so I'm good since I bought it about 7 months ago. The warranty process is very streamlined at Maxtor - completely hands-off and web-based. I was given the choice of paying $20 for expedited replacement (two-day shipping), or a free replacement that could take a couple of weeks (I would have to send in my disk first, then get one back through slow mail).
I like my backups, so I chose the $20 option, and the disk arrived 3 days later (submitted the form on Sunday night, it was processed and shipped on Monday, and arrived on Wednesday).
It took all night (about 10 hours) to re-sync my Time Machine backups. About 1,256,435 files in 600GB of data. I was quite surprised to see that I've got over a million files... Hmm.
Ah, I feel safe again.