My drive started to fail in my Dell PowerEdge 400SC, so Dell shipped me a new drive. I installed it and Dell Support told me to boot-up and repeatedly press Ctrl-Alt-D. Apparently this runs some sort of built-in BIOS hard drive diagnostics.
In my case, it happened to print out "Fail. Return Code: 4". The tech explained that this means "electrical failure".
I sent back the two bad drives, Dell shipped me a new one and it seems to be running fine. If this happens again on another Dell, I'll be sure to run Ctrl-Alt-D to start...
UPDATE: Looks like this is briefly documented here. I didn't find a list of return codes, but Return Code 7 seems to mean "bad tracks or sectors".