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.)
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