Two different jobs
RAID or a mirror distributes data across drives so a failed drive may not stop access. A backup is a separately recoverable copy. Both can be useful, but neither substitutes for the other.
What array protection cannot solve by itself
- Accidental deletion and unwanted overwrite.
- Ransomware or corruption that replicates across the array.
- NAS theft, fire, water, or a power event affecting the system.
- Configuration mistakes or failed recovery procedures.
Practical method
- Choose availability protection with the RAID Protection Decision Tool.
- Allocate one separately stored local copy and one offsite or otherwise independent copy for important data.
- Schedule a restore test and record who can perform it.
Scenario
A four-drive array can survive a permitted drive failure, but it cannot restore a folder deleted last week unless an independent version or copy exists. The recovery plan must name that copy and how it will be tested.
Limits
There is no universal copy count or retention period. The correct plan depends on data value, recovery time, locations, and the ability to test restores.
Related: RAID Capacity Calculator and Storage and RAID Capacity Formulas.