Disk failures

I said it was a prehistoric system, and indeed anything more modern than 
about, what, 1991 won't have this problem.

I'm not sure if a head failure could cause a failure to find sector 
address markers: I'm not sure if you could even distinguish the two cases 
without digging into the drive. (As I said, my expertise in hard drive 
engineering is notable mainly by its absence.)

It's just that heads are solid-state, and solid-state stuff doesn't die 
all that often, while the head assembly itself is being wrenched all over 
the place: simple bending could explain this, I think.