[X4U] Mac mini server drive failure

[Joe Sporleder]

I have a Mac mini server (previous generation) with the internal hard drives setup in a mirror RAID. While doing my normal Monday night backup routine, Disk Utility was telling me that the RAID had degraded, and clicking on more detailed kept showing me that the drive in the "upper" bay had failed as part of the RAID set. 2 attempts at rebuilding the set failed, as well as erasing everything, creating the mirrored RAID from scratch and restoring from backup, still resulted in a failed RAID. 

This tells me that the Hitachi drive in the "upper" bay has bit the bucket. Both drives are 5400 RPM, 500GB. I have a couple of spare Seagate Momentus 500GB drives running at 7200 RPM. Any problem with replacing the bad one with a 7200 RPM drive, or do both need to be the same speed of drive? Or, should I pull the good Hitachi drive too, and replace both with the Seagates?

Joe