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RAID on a NAS can keep an array available after a limited number of drive failures, depending on its configuration. It does not protect against every cause of data loss and is not a backup. A separate backup is still needed for files that are deleted or corrupted, or when the NAS itself is lost.
What RAID protects against
RAID combines drives into an array with a defined level of redundancy. Depending on that level, the array can continue operating after one or more member drives fail. That tolerance is a design limit, not a promise that data is safe from other failures. Synology’s RAID guidance describes RAID 0 as having no data redundancy, RAID 5 as tolerating one failed drive, and RAID 6 as tolerating two.
RAID choices involve trade-offs among usable capacity, drive-failure tolerance, and performance. The exact features and drive requirements depend on the NAS model and its implementation; a RAID label alone does not describe the protection plan for all your data.
What different RAID levels tolerate
| RAID level | Drive-failure tolerance | What that means |
|---|---|---|
| RAID 0 | None | It provides no data redundancy, according to Synology’s RAID overview. |
| RAID 5 | One drive | Synology says RAID 5 uses one parity block and can sustain one drive failure. A second drive failure before the array is rebuilt can result in data loss. |
| RAID 6 | Two drives | Synology says RAID 6 uses two parity blocks and can sustain two drive failures. |
| RAID 10 | Depends on which drives fail | RAID 10 stripes data across mirrored pairs. It can tolerate one failed drive in each pair, but not the failure of both drives in the same pair, as described in Seagate’s RAID-level documentation. |
These are design tolerances, not estimates of how likely a failure is. The sources do not establish general probabilities for NAS drive failures, rebuild failures, or multiple drives failing together.
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What happens when a NAS drive fails
If a member drive fails and the array is still within its tolerance, the NAS may continue operating in a degraded state: its protection is reduced until redundancy is restored. Rebuilding means restoring that redundancy after replacing the failed drive. The process can affect array performance. On RAID 5, Seagate warns that a second drive failure before the rebuild completes can mean loss of array data. See Seagate’s RAID Manager documentation.
- Check the array status. Use your NAS management interface to confirm which drive or array is reporting a problem; consult the instructions for your specific NAS model.
- Confirm a replacement drive is compatible. Check the NAS model’s current drive compatibility list and requirements. There is no single replacement drive established as compatible with every NAS.
- Replace the failed drive according to the NAS maker’s procedure. Avoid making other array changes while protection is reduced.
- Allow the rebuild to finish and verify the array returns to a healthy state. Redundancy is not restored until rebuilding is complete.
Seagate also advises backing up important files before making changes to an existing array in its RAID Manager guidance.
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What RAID does not protect against
RAID redundancy addresses a limited class of drive failures; it does not preserve a separate recoverable copy of files. Synology states directly that “RAID (classic and SHR) is not a backup solution” in its SHR overview.
- Accidental deletion: Redundancy does not provide an independent copy from which to restore a file that was removed.
- Corruption: A redundant array is not the same as a versioned backup that can restore an earlier good copy.
- Loss of the storage system: If the NAS or its array is lost, RAID alone does not provide another copy elsewhere.
- Failures beyond the array’s tolerance: A further drive failure while protection is reduced can cause data loss, depending on the RAID level and where the failure occurs.
How to pair RAID with backups
Use RAID for continuity through the specific drive failures its level can tolerate, and backups for recovery when files or the storage system are lost. Synology recommends the 3-2-1 approach: “3 copies; 2 storage mediums; 1 offsite.” Its backup guidance describes local multi-version copies as useful for recovering from accidental deletion or corruption, while warning that RAID crashes and hard-drive failures can still cause data loss even when such a copy exists.
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- Keep multiple copies of important data, rather than treating the RAID array as one copy plus a backup.
- Use more than one storage medium.
- Keep at least one copy offsite so it is separate from the NAS and local backup destination.
An external drive can be one possible local backup destination, but it does not by itself fulfill the offsite part of the 3-2-1 recommendation.
Quick Recap
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- Powerful Pass-Through Charging: Supports up to 85W pass-through charging so you can power up your laptop while you use the hub. Note: Pass-through charging requires a charger (not included). Note: To achieve full power for iPad, we recommend using a 45W wall charger.
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