When I was in college, interning with the Social
Security Admin in 1982, I visited a huge disk farm.
Easily 200 dishwasher sized floor standing units in
a vast array stretching across a large room. As my
guide was giving me his canned talk, I saw something
strange out of the corner of my eye. I interrupted
him and he told me to just keep scanning across the
units. Every minute or so, one of the drives would
puff out a small orange cloud. I was told that each
puff was a head crash. The drives would expel the
orange oxide that had been scraped from the surface,
the head would almost always be undamaged and the
drive would keep right on going. The bad blocks
would be noticed and marked by the OS and when a pack
had enough bad blocks it would be pulled and sent out
to be resurfaced.