You should use the exact same size and make and model for optimial performance. They need to use the same controller at the very least. If you use one controller that is compatible but slower, then it will be only as fast as the slower one in most cases. If the system does use the faster of the two, then it will get very hot and wear faster. Since the controller is almost always integrated with the HD (at least the hardware end of it) and the other half, built in to the MB, then it stands to reason that performance is degraded for too many reasons to list here. Raid was designed for fault tolerance, Raid 1 Mirroring , or speed Raid 0 Striping. Both are good reasons to actually use it.