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RAID on a NAS can keep an array available after a limited number of member-drive failures, depending on its RAID level. It is not a backup: it does not provide a separate recoverable copy if files are deleted or corrupted, or if the storage system is lost. Synology explicitly says classic RAID and SHR are not backup solutions.
What RAID protects against
RAID combines drives into an array using a particular layout. Some layouts include redundancy, so the array can continue operating after a specified number of drives fail. That tolerance is a design limit, not a guarantee against every cause of data loss.
RAID choices involve trade-offs among usable capacity, drive-failure tolerance and performance. Exact features and limits can vary by NAS model and implementation, so check the documentation for your device rather than treating a RAID label as a complete protection plan.
How much drive failure can different RAID levels tolerate?
| RAID level | Drive failures tolerated | What to know |
|---|---|---|
| RAID 0 | None | Synology describes RAID 0 as having no data redundancy. |
| RAID 5 | One | Synology says RAID 5 uses one parity block and can sustain one failed drive. |
| RAID 6 | Two | Synology says RAID 6 uses two parity blocks and can sustain two failed drives. |
| RAID 10 | Depends on which drives fail | Seagate describes RAID 10 as mirrored pairs striped together. It can tolerate a failure in each pair, but not failures of both members of the same pair. |
These are design tolerances, not estimates of how likely a failure is. The vendor guidance cited here does not provide a general NAS failure probability or a percentage chance of another drive failing during a rebuild. Synology’s RAID 5 and RAID 6 figures come from its Knowledge Center guidance; Seagate’s RAID 10 explanation is from its RAID documentation.
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What happens when a NAS drive fails?
If a drive fails but the array remains within its RAID level’s tolerance, the NAS may continue operating in a degraded state. Degraded means the array has less protection than it had before the failure. Replace the failed drive with one compatible with the specific NAS, then allow the system to rebuild redundancy. Seagate notes that rebuilding can affect array performance.
Until the rebuild is complete, another failure can exceed the array’s tolerance. For example, Seagate warns that a second drive failure in a RAID 5 array before rebuilding finishes results in lost array data. RAID 10’s exposure depends on the failed drives’ mirror-pair locations: a second failure may be tolerated if it is in a different pair, but losing both drives in one pair is not.
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Before replacing a drive or making changes to an existing array, consult the NAS vendor’s instructions. Seagate’s RAID Manager documentation advises backing up important files before changing an array. Check the NAS model’s compatibility list and drive requirements; there is no universally compatible replacement drive established by the cited guidance.
Why RAID is not a backup
RAID redundancy and backup solve different problems. RAID is intended to sustain the array through a limited class of drive failures. A backup is a separate copy that can help recover files after accidental deletion or corruption, or if the storage system is lost. If a file is deleted or corrupted on a RAID array, RAID alone does not create an earlier, clean version to restore.
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Synology recommends the 3-2-1 approach: “3 copies; 2 storage mediums; 1 offsite.” A local multi-version copy can help recover from accidental deletion or corruption, but Synology cautions that RAID crashes and hard-drive failures can still cause data loss even when such a copy exists. An external drive can be one local backup destination; it does not, by itself, supply the offsite copy in that recommendation.
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- Choose a RAID level based on its drive-failure tolerance, usable capacity and performance trade-offs for your NAS.
- Keep a separate, versioned backup so you can restore deleted or corrupted files.
- Include an offsite copy as part of a 3-2-1 plan.
- Verify replacement-drive compatibility for your exact NAS model before buying.
- After a drive fails, treat the array as having reduced protection until rebuilding is complete.
Synology’s RAID-level and SHR guidance applies to the DSM product families listed in its documentation; Seagate’s rebuild and RAID 10 explanations describe its own documentation and implementation. Check your NAS manufacturer’s current instructions for model-specific behavior.
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