getting a "degrated" message on one of my dual hard drive that came with my xps420

It came with a "Serial ATA2 Dell DataSafe WithDual 500GB Hard Drives", I'm getting an error message saying one of them is "degraded".

I'm assuming I'll have to replace it, but, how do I know which one is no good?

what do I have to do?

what am I looking for when shopping for a hard drive so that it is compatible?