Rebuilding a failed RAID 5 drive to a spare drive. Second one in two weeks on the same array... looks like they're starting to hit the end of their usable lives.
Make sure your RAID enclosure's ventilation is working the way that it should. The last time I had two drives go bad in one array, the (likely) culprit turned out to be a dead fan.
I'm waiting for that to start happening with my RAIDs, as I bought all of the drives at the same time. I do have one RAID backup to another near identical unit, though, and then they are also backed up to external drives..