My mum's ancient 486 laptop had a really strange disk failure this
Christmas. It started with a single bad sector, but then within about
fifteen minutes one third of the sectors on the disk (in contiguous runs
of varying length) were returning, not bad sectors, but `sector not
found', i.e. the drive couldn't even find the sector address markers.
What I suspect may have happened, based on my extensive lack of experience
in hard drive design, is that all the G forces the head assembly is
exposed to whenever a seek happens had over time twisted the head reading
the farthest side of whichever platter didn't contain the servo track out
of true, so that when the servo track said it was over track X, the
topmost heads were actually midway between tracks or something like that.
In that position they couldn't read the sector addresses, couldn't find
any data, and whoompfh, goodbye data.
(I've never heard of this failure mode anywhere else, and perhaps it was
something different, but still, it was very strange. Disks *can* go mostly
bad all at once. It's just rare.)