Out of curiousity:

Are the drives of the same brand?
What is the standing temp?

I've seen an entire batch of Maxtor IDE's come out bad just as gallant and KarlZimmer mentioned. It was a fault in the ball bearings from what we could figure out.

Heat will cause horrible things to happen to drives as well...Make sure if you are running raid arrays to do EVERYTHING possible to keep the machines and specifically the drives well ventilated.

Going as far as to keep those systems as close to the floor as possible is not a bad idea either.