Well if the hard drive doesn't spin up fast enough then the head can't read the data from the plate. I would figure a guess that the leading cause of dead drives is an old motor that just can't spin fast enough to read anymore. However, it will still try to spin up so it would sound like it's working.
My hard drive is starting to make a "clack" sound, though, like something is dropping in it. Harddrives usually lightly click, but this is like "click...click.. cliCLACK" The same thing happenned to a drive at work the day before it died on me.

I figure I've got a couple days to move everything before my MP3 collection is toast (again). I already had to rebuild it once from a HD crash...
