Comparing usable capacity
For N drives of capacity S, RAID 5 uses (N − 1) × S and RAID 6 uses (N − 2) × S. This model requires at least three drives for RAID 5 and four for RAID 6.
| Item | Value |
|---|---|
| 4 × 4 TB | 16 TB |
| RAID 5: (4 − 1) × 4 | 12 TB |
| RAID 6: (4 − 2) × 4 | 8 TB |
Assumptions: equal drives, theoretical capacity before formatting and reserves.
Choosing beyond capacity
RAID 6 provides less usable space but keeps redundancy against a second drive failure. Also consider the hardware, writes and rebuild time; the table does not predict performance.
Frequently asked questions
Does RAID replace a backup?
No. Disk fault tolerance does not restore deleted files, logical corruption or incidents affecting the whole array. Keep separate backups and test restoration.