Why is the disk capacity of the NAS is reduced after RAID configuration completes?
There are a couple of possible reasons.
a. When configuring two disks as a RAID 1, the disk size usable is equal to the size of one disk.
When configuring 'N' disks as RAID 5, the disk size usable is equal to the size of (N-1) disk(s).
b. Hard disk manufacturer calculates disk size as 1M=1000,000 while the computer calculates actual disk capacity as 1M=1024x1024.
When a manufacturer marks a disk size as 2TB
(disk size: 2TB=2,000GB=2,000,000MB=2,000,000,000KB=2,000,000,000,000),
the actual usable disk capacity could be 2,000,000,000,000/1024/1024/1024/1024 = 1.81TB.
Therefore when configuring 5 x 2TB disks as RAID 5, the total capacity will be 1.81TB x (5-1)=7.24TB