When the disks start to whine…[update: replace 'em.]
The noise from the disk from which these very bits were read has suddenly jumped a few decibels. More than a few, in fact. And at a new pitch that is almost painful even emanating through the closet door.
Sudden, catastrophic failure seems possible. Plausible, even.
Don't say you haven't been warned. If it all goes down, it'll all come back up again…eventually.
[Update: On my trip out to get new ink cartridges for my printer, I saw Maxtor 80Gb drives on sale for $60. With a $15 coupon in my pocket, that looked like an unbeatable deal. So the machine was down, and now its back up, and I think it's working. I've got an upgrade story to tell, with pictures even, but that'll have to wait until I have more time.]