RAID 5 and RAID 6 both use distributed parity, but RAID 6 adds a second parity block — and on modern high-capacity drives that difference is decisive. Here is how they compare, and you can run both on your own drives in the RAID calculator (5 vs 6).
The core difference
RAID 5 keeps one parity block per stripe, so usable capacity is (n−1) × drive size and the array survives a single drive failure. RAID 6 keeps two independent parity blocks, so usable capacity is (n−2) × drive size and it survives any two failures. You trade one drive of capacity for a second layer of protection.
On eight 16 TB drives, RAID 5 gives 112 TB usable (87.5%) and RAID 6 gives 96 TB (75%). The calculator shows both instantly.
Rebuild safety — the deciding factor
When a drive fails, the array reads every surviving drive to rebuild the replacement. In RAID 5 there is no redundancy left during that read, so an unrecoverable read error (URE) on a surviving drive means data loss. On large nearline drives that probability is real — see is RAID 5 dead?.
RAID 6 still has a second parity during a single-drive rebuild, so a URE is reconstructed and the rebuild continues. This is the single biggest reason RAID 6 is the default for large-capacity HDD arrays.
Performance and cost
RAID 5 has a ×4 write penalty; RAID 6 has ×6, because it must update two parities. For read-heavy and sequential workloads the difference is small; for write-heavy workloads, neither is ideal and RAID 10 is better.
RAID 5 is cheaper per usable TB (one parity drive vs two). On small arrays of modest drives that can be acceptable; on large arrays the extra parity drive is cheap insurance against a failed rebuild.