@Jacka: What kind of setup is this---a custom-built PC, an OEM PC, or a server? It sounds like an OEM.
As a matter of curiosity: Even if you can't find the same exact model, can you find a drive with the same capacity?
@ITGuy24: Nitpick(?) on your comment up top---this is a RAID 1 or mirrored array.
I concur. If you use a larger drive in a generic array (i.e. not using specialized software like MirrorFolder or FlexRAID), you shouldn't have a problem, but: (1) you do lose the extra capacity, and (2) the other drives generally have to "wait" in some way for a lower-performance drive. Note that equal RPMs does not mean equal performance. Cache size plays a huge role, and access times perhaps a slight one.
(Sorry, could only post one link, but FlexRAID is easy to find.)