What each level trades
RAID combines several drives into one array, trading capacity for speed, redundancy, or both. The five common levels sit at different points on that trade-off:
| Level | Usable (n drives, size s) | Survives |
|---|---|---|
| RAID 0 | n·s (striped) | nothing — any failure loses all |
| RAID 1 | s (mirrored) | up to n−1 drives |
| RAID 5 | (n−1)·s | 1 drive |
| RAID 6 | (n−2)·s | 2 drives |
| RAID 10 | (n/2)·s | 1 per mirror pair |
Mix drive sizes and the parity and mirror levels can only use the smallest drive's worth from each — a 4 TB drive in an array of 2 TB drives contributes just 2 TB, and the rest is wasted. RAID 0 is the exception; it simply sums.
TB versus TiB
Drive makers sell terabytes as 10¹² bytes, but operating systems usually show tebibytes (2⁴⁰ bytes). That is why a "12 TB" array reports as 10.9 TiB — no space is missing, the two units just differ by about 9%. This calculator shows both.
The rebuild-risk argument, honestly
When a drive fails in a parity array, rebuilding onto a replacement means reading every remaining drive in full. Drives have a specified unrecoverable read error (URE) rate — often one bit in 10¹⁴ for consumer disks. Reading a 12 TB RAID 5 rebuild is about 9.6×10¹³ bits, giving a worst-case URE probability of 1 − e^(−0.96) ≈ 62%. That alarming figure is behind every "RAID 5 is dead" article.
The caveats matter, though. The URE spec is a conservative floor; real drives routinely beat it by an order of magnitude. Many UREs land in unused space or are recoverable at the filesystem layer. And enterprise drives are rated at 10¹⁵ or better, which drops the same rebuild risk below 10%. What survives all the caveats is the practical conclusion: for large arrays, prefer RAID 6 (its second parity block absorbs a URE during a single-drive rebuild), keep arrays from growing without bound, and remember that RAID is not a backup — it protects against drive failure, not deletion, corruption, or fire.
Related tools: the data transfer time calculator estimates how long a rebuild actually takes, and the subnet calculator is the other everyday homelab tool.