By default, Windows displays hard drive capacity using binary numbers as opposed to decimal numbers. A 6TB hard drive will be reported by Windows as 5.58TB.
RAID 5 usually reserves around 20%-25% of all accumulated storage for parity.
“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.”