RAID 10 with six 4 TB drives: three mirror pairs and the odds that matter

The guaranteed number is 1 drive, and that's the worst case. Once one drive dies, only its mirror partner is fatal, since the partner holds the only other copy of that pair's data. The other four drives sit in pairs that are still whole, so losing one of them is survivable.

A second failure is survivable 80% of the time, if each of the five drives left is equally likely to fail. Four of those five sit in intact pairs. Failures stack the same way, up to a point. Three drives can fail, but only if each one hits a different pair. Four failures always double up one pair, so without a rebuild in between, the array is gone.

Drop to four drives and the odds fall. Two pairs leave three drives after the first failure, one of them a partner, so a second failure is survivable only 66.7% of the time.

Rebuilds stay simple. The replacement drive copies its mirror partner, so one 4 TB drive gets read, not the whole array. On a quiet array that's a best case of 7 hours. Real rebuilds run slower when the array is busy, and the degraded pair is the only one with no copy left until the copy finishes.

RAID 6 on the same six drives is the other way to spend them. It gives 16 TB usable, more than this layout's 12 TB, and it survives any two failures wherever they land. RAID 6 guarantees two failures where RAID 10 guarantees one. RAID 6 writes are slower too, because every stripe needs two parity blocks computed and written, while RAID 10 writes go straight to two copies.

You'll find this layout in six-bay servers, VM hosts and database servers, where random writes are heavy and raw capacity comes second.

Choose RAID 10 with six 4 TB drives for write-heavy work like databases and VM hosts. You get 12 TB usable from 24 TB raw, a 50% overhead, and a rebuild that copies one drive from its partner. The price is a guarantee of one failure, with 80% odds on the second.

Choose RAID 6 on the same six bays when capacity and a two-failure guarantee matter more than write speed. It gives 16 TB from six bays and survives any two failures.

Either way, RAID isn't a backup. Deleted files, ransomware and a failed controller hit both layouts.