The rule to remember
RAID improves availability; it does not create an independent backup. Accidental deletion can be mirrored immediately.
Quick comparison
| Level | Minimum | Typical tolerance | Common reason |
|---|---|---|---|
| RAID 1 | 2 | 1 drive | Simple small NAS |
| RAID 5 | 3 | 1 drive | Capacity efficiency |
| RAID 6 | 4 | 2 drives | More rebuild margin |
| RAID 10 | 4, even | Depends on mirror pairs | Performance and faster rebuilds |
Large drives increase rebuild exposure
A rebuild reads the surviving drives for a long time. For irreplaceable data, compare the extra margin of RAID 6 or an independent backup with the capacity you give up.
A safer decision order
- Identify data that cannot be recreated.
- Fund an independent backup first.
- Set acceptable downtime and drive-failure tolerance.
- Then compare usable capacity.
Sources and official tools
Before you buy: Check the compatibility list and calculator published by your NAS, NVR, or drive manufacturer.