No, but there isn't much of a performance difference between the two. RAID 5 uses the third disk for parity which can be used to rebuild either of the two data disks if one fails. It's basically a hybrid of the way both a RAID 0 (striping) and RAID 1 (mirroring) concepts. If you put 3 500GB HDDs in RAID 5 you will have a 1TB array with complete data integrity unless 2/3 drives die at the same time.