A 6TB hard drive would be reported by Windows as 5.46TiB.
RAID 5 reserves the capacity of one drive for parity purposes. The available storage space of a 5 x 6TB RAID 5 array corresponds to 4 x 6TB.
4 x 6TB = 24TB = 21.8TiB
The minumum number of drives in a RAID 5 configuration is 3, in which case parity storage consumes 33% of the total capacity.
http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5
“RAID 5 consists of block-level striping with distributed parity. … It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.”
