RAID 0+1 combines the performance of RAID 0 with the data protection of RAID 1 by
mirroring a striped set on a second pair of drives. Because mirroring duplicates all data,
this level offers less usable disk space than RAID 5. Usable space is half of the total
space available on the drives in the set. You need four disks to create a RAID 0+1 set.