OT: I heard that the disks in mainframe hard drives used to be gigantic and that, when one failed and went off the spindle, they'd actually bust through the housing where they were held and roll out. Hence, "hard drive crash". Dunno if that's true or not...
That's a myth too - although theoretically possible with early drum memory units - some of those drums were massive, and had an immense amount of rotational energy. Most disks weren't really big enough (14" in diameter is common). A main paging drum, however, could be 12 feet long...
A true hard disk crash happens when the heads hit the media. And yes, I've heard of insects being involved here. Many old minicomputer/mainframe drives were removable. That is, the disk pack (stack of disk platters) was changed in and out of the drive (containing the motor, heads, etc). They were very susceptable to dust, dirt, and any other contaminents, seeming as though the heads fly very closely above the media. Crashes happen any time the head hits the media, or something on it. At the high speed, any head/media contact is bad. Usually it'll gouge some oxide off the disk and clog up the head. A classic story (that has indeed happened many times), is where a disk develops a crash for some reason or another and goes offline. The operator, thinking a the disk pack might have gotten corrupt, swaps in another one and tries to load it. The drive spins up, loads the heads, and crashes the contaminated head into the media, ruining it. The drive still isn't working, so the operator tries the pack in another drive - the good heads hitting the existing gouge in the media, contaminating them. A stupid operator can continue swapping disks until a majority of the drives in the room are crashed, and ruining every good pack loaded into them. This has most definitely happened.
The initial cause can be just about any contaminent, and I've heard stories of the culprit being an ant or other small insect found smeared into the platter, but at the same time, at those speeds, I don't know how you'd be able to identify the insect - there wouldn't be anything left.
Crashes did happen, usually they aren't catestrophic, but can result in nice grooves gouged into the media. The fix is to replace the heads in the drive and discard the pack.
Drive crashes still happen, it's just that modern drives are fairly sealed, so it's usually caused by bearing failure or as a result of dropping the drive. I've seen modern IDE disks full of aluminum shavings with huge gouges in the media. I have a 9" platter from a CDC 496mb drive on the wall in my cube with some deep grooves cut in it from a drive crash at a customer's site when someone walked on the wiring behind the computer and bounced the power to the disk cabinet.
-Ian