Mac Pro RAID Card: Capacity for four-drive RAID 5 configurations may be different than expected
When a Mac Pro RAID Card is used for a four-drive RAID 5 configuration, the available capacity may be lower than expected due to data placement and layout techniques. In this configuration, the available capacity is approximately 66% of the total capacity of all four hard drives.
Note: This rule does not apply to RAID 0, RAID 1, or the use of three hard drives in a RAID 5 set.
Consult the table below to determine available capacity for various RAID levels using four hard drives.