I setup a FlexRAID storage pool with DRU1 (1x 4 TB HDD), DRU2 (2x 2 TB HDD), DRU3 (1x 4 TB) and 1 PPU (4 TB). Recently, one of the 2 TB HDDs in DRU2 failed. I did a swap-and-restore operation, but FlexRAID reported an error: "too many failed devices = 2" and the maximum of failed devices would be 1. The second disk in DRU2 works just fine. What did I do wrong? Did I get the whole concept wrong?