The Tool Desk
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Use RAID for availability and a separately managed backup for recovery. Protect that backup from the same threats as the array, keep an offline or off-site copy where practical, and regularly prove that files can actually be restored.
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What RAID does—and what a backup does
RAID combines multiple drives under one storage system. Depending on the level and implementation, redundancy can let the array continue operating after a limited number of drive failures. That reduces downtime and can give you time to replace a failed drive and rebuild the array.
A backup is a separate copy stored outside the primary failure domain. Seagate describes examples such as an external hard drive, optical media, another local NAS, or cloud storage. The copy must be independent enough that a problem affecting the array does not automatically destroy the copy as well.
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| Capability | RAID | Backup |
|---|---|---|
| Keep files available after a supported disk failure | Often, depending on RAID level and implementation | Not its primary purpose |
| Recover an accidentally deleted file | Usually no; deletion is normally duplicated across the array | Yes, if the backup has a usable earlier version |
| Recover from corruption or malware encryption | Usually no; corrupted or encrypted data can be replicated across members | Yes, if an unaffected historical copy exists |
| Protect against theft, fire, flooding, or site loss | No, when all array members are in the same enclosure or location | Yes, if a copy is physically or geographically separate |
| Reduce downtime while replacing a failed drive | That is a core benefit | May provide recovery, but does not keep the original array online |
RAID and backup can therefore coexist: RAID supports continuity, while backup provides a route back to an earlier or independent state after broader loss.
Why redundancy does not cover common data-loss events
Accidental deletion and unwanted changes
If a user deletes a folder from a mounted RAID volume, the array generally records that deletion as a valid operation. Mirroring or parity does not know that the change was a mistake. A backup with version history or a retained snapshot can provide an earlier copy; the RAID volume alone usually cannot.
File corruption and application errors
Applications can overwrite files incorrectly, databases can become inconsistent, and an undetected error can spread through synchronized storage. Redundancy helps a drive continue operating, but it is not a historical record of every good version. Recovery depends on having a separate copy created before the corruption occurred.
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Malware and ransomware
Ransomware can encrypt files that are accessible to a user or service account. If the backup destination is continuously mounted with write access, the same attack may reach it. CISA’s September 2023 #StopRansomware Guide recommends offline, encrypted backups and regular tests of their availability and integrity.
Failures beyond an individual disk
A RAID array can depend on a controller, backplane, power supply, firmware, enclosure, operating system, or network path. Seagate notes that RAID does not protect against all hardware failure. A controller or enclosure problem can make every member inaccessible even when the disks themselves are healthy.
Theft, fire, flooding, accidents, and site-wide events
When all drives and the backup are in one room, a fire, flood, theft, or other site event can remove both. CISA identifies device loss or theft, wear, accidents, and malware among data risks. Physical separation addresses a different failure domain than RAID redundancy.
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Failure tolerance depends on the RAID level
There is no universal “RAID protection” number. The level, implementation, drive count, controller, and rebuild state determine how many failures the array can withstand. In Seagate NAS OS guidance, RAID 5 is described as tolerating one failed disk and RAID 6 as tolerating up to two. Those are examples from that documentation, not a guarantee for every NAS or controller.
| Example in Seagate NAS OS guidance | Documented disk-failure tolerance | Important qualification |
|---|---|---|
| RAID 5 | One failed disk | A further failure before protection is restored can result in data loss |
| RAID 6 | Up to two failed disks | The limit applies to that implementation; other systems may differ |
Once failures exceed the level’s tolerance, the array may be inaccessible or data may be lost. Even before that point, operating with a failed drive leaves less margin for another problem. Treat the documented tolerance as a maximum for a specific configuration, not as permission to postpone a backup.
What “degraded” means and why rebuilds are risky
A degraded array has lost part of its redundancy. Seagate RAID Manager documentation says protection is limited until failed drives are replaced and the array is rebuilt. The files may remain readable, but the system is no longer in its normal protected state.
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Rebuilding reconstructs missing data onto a replacement drive. It can consume substantial storage and controller resources, affect performance, and expose the array to additional risk while redundancy is being restored. A second failure during that window may exceed the configuration’s tolerance.
Do not apply a generic repair recipe to every NAS. Controller behavior, drive order, hot-spare rules, replacement requirements, and recovery options vary. Follow the exact NAS or controller instructions, and preserve any remaining readable data before making changes that could worsen the situation.
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Build a backup design independent of the array
Choose the destination and operating process by considering the full failure domain, not just capacity. The following questions expose gaps that a successful-looking backup job can hide.
- Independence: Can the array, controller, enclosure, credentials, or network failure affect the copy at the same time?
- Ransomware resistance: Is at least one copy offline, encrypted, immutable, or otherwise protected from unauthorized changes?
- Physical separation: Would a fire, theft, flood, or other site event destroy both the primary and the copy?
- Retention: How far back must you recover, and how many historical versions are needed?
- Restore performance: How quickly can you locate and restore a file, folder, or entire service?
- Operational effort: Who receives failure alerts, changes media, checks capacity, and performs test restores?
- Cost and capacity: Does the destination hold the current data plus the selected history, with room for growth?
Use the 3-2-1 framework as a starting point
Backblaze describes the 3-2-1 approach as three copies, on two types of media, with one copy off-site. It is a planning mnemonic rather than a measured guarantee or a complete retention policy. Adapt it to your data’s change rate, geographic risks, budget, and acceptable recovery time.
For example, a NAS might be the working copy, an external drive might provide a second local copy, and an encrypted off-site or offline copy might address a site-wide event and ransomware exposure. The exact arrangement is yours to set; the important properties are separation, recoverability, and regular verification.
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Practical steps to protect a RAID array
- Document the array. Record the RAID level, member drives, usable capacity, controller or NAS model, encryption settings, applications that depend on it, and the location of recovery instructions. Keep this information somewhere that remains available if the enclosure is lost.
- Identify what must be recoverable. Include documents, photos, databases, virtual machines, configuration exports, credentials stored for recovery, and any data that cannot be recreated. Separate irreplaceable files from disposable downloads so retention and testing reflect their value.
- Set recovery objectives. Decide how much recent work you can afford to lose and how long the service can be unavailable. No source cited here prescribes one interval or retention period; those choices depend on how quickly your data changes and the consequences of downtime.
- Select an independent target. Seagate recommends backing NAS data to direct-attached storage or another NAS. An external hard drive is a concrete local option; a second NAS or cloud/off-site destination may fit larger or distributed environments. Verify compatibility and capacity for your particular system.
- Configure versioned backups where needed. A single mirror of the current state will reproduce deletions and corruption. Retain earlier versions for data that may need rollback. Choose the history length based on your recovery objectives rather than assuming that one successful copy is enough.
- Protect the destination. Use encryption for sensitive data. Keep at least one copy disconnected or otherwise resistant to writes when it is not being updated, following CISA’s offline and encrypted backup guidance. Restrict accounts and permissions so an incident on the NAS cannot automatically erase every copy.
- Create physical or geographic separation. Move an appropriate copy outside the room or site that holds the array. An off-site copy addresses fire, theft, flooding, and other local disasters that a second device in the same rack does not.
- Monitor jobs and capacity. A scheduler reporting “success” does not prove that every required file was included or is restorable. Review failures, skipped files, authentication errors, disconnected media, and capacity warnings.
- Test restoration. Restore representative files, folders, permissions, and application data to a separate location. CISA specifically recommends regular tests of backup availability and integrity. Record what worked, how long it took, and which steps need updating.
- Revisit the design after changes. Adding data, changing the RAID level, replacing a NAS, altering encryption, or moving sites can invalidate assumptions about capacity, access, and recovery. Update the documentation and perform another restore test.
Choosing a backup destination
| Destination category | Useful when | Strengths | Risks and checks |
|---|---|---|---|
| External hard drive | You need a straightforward local copy for a small or moderate dataset | Portable, simple, and directly controlled by you | A continuously connected drive can share ransomware and accidental-deletion exposure; disconnect or otherwise protect it when not updating, and store a separate copy for site loss |
| Second NAS | You need centralized storage for another room, building, or managed location | Can support scheduled transfers and larger retained histories | Compatibility, permissions, network paths, and configuration are system-specific; a second NAS at the same site may not survive a site-wide event |
| Offline removable media | You need a copy isolated from routine network attacks | Disconnected media is harder for network malware to modify | Media must be labeled, stored securely, checked for readability, and rotated so one copy is not the only historical version |
| Off-site or cloud backup | You need protection from local disaster or want geographic separation | Addresses theft, fire, flooding, and other site-wide events when configured appropriately | Account security, encryption, bandwidth, retention, restoration time, and provider-specific behavior must be evaluated for your data |
No source establishes a universally best medium, capacity, provider, price, or schedule. Size the destination for your data and retained history, then compare independence, offline or immutable protection, separation, restore speed, cost, and the work required to operate it.
How to handle a degraded array
When the data is still readable
- Confirm which drive or component the controller reports as failed; do not remove a healthy member based only on physical position.
- Start or verify a separate backup of important files before making nonessential changes.
- Capture current alerts, logs, configuration details, and the documented recovery procedure.
- Obtain a compatible replacement according to the controller or NAS requirements.
- Follow the system’s rebuild procedure and monitor progress, performance, and new errors.
When another failure occurs or files become inaccessible
Stop improvising. Further drive swaps, initialization, or forced rebuilds can change recoverability. Preserve the remaining media and consult the exact platform documentation or a qualified data-recovery professional. Your independent backup should be the primary recovery route; the degraded array is not a substitute for it.
Common backup mistakes
- Calling a mirror a backup: A synchronized copy can reproduce deletions, corruption, and encryption immediately.
- Keeping every copy in one enclosure: RAID members and a local backup can disappear together through theft, fire, flooding, or enclosure failure.
- Leaving the only backup permanently mounted: Malware or a compromised account may have permission to alter it.
- Checking only job status: Logs can show completion while a required share, database, or permission set was excluded.
- Ignoring retention: Without earlier versions, you may discover corruption after the only good copy has been overwritten.
- Never practicing recovery: A backup that has not been restored is an assumption, not demonstrated protection.
- Waiting until rebuild begins: A degraded array has reduced protection; delaying backup until after replacement increases exposure.
A simple decision test
Ask what would happen if someone deleted a critical folder, ransomware encrypted the NAS, the enclosure failed, and the building became inaccessible. If the answer is “the RAID should handle it,” the design covers only a narrow disk-failure scenario. If the answer identifies an independent, protected copy, a retained earlier version, a physically separate location, and a tested restoration procedure, the array is being used for availability while backup handles recovery.
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