When a drive fails in a redundant RAID array, the system usually marks the array degraded and keeps serving data from the surviving drives while it rebuilds onto a replacement or spare. During that recovery, redundancy is reduced and performance may suffer. Whether data stays available—and whether it can be reconstructed—depends on the RAID layout, the condition of the remaining drives, and the failure pattern. A high-capacity drive can make the degraded period longer, but there is no reliable universal rebuild-time estimate.
What happens after a drive fails
- The storage system detects a missing or faulted member. A redundant array may continue operating in a degraded state; one without enough surviving copies or parity may become faulted. OpenZFS, for example, distinguishes online, degraded, and faulted pool states in its pool-state documentation.
- It reconstructs missing data if the layout allows. A controller or storage system reads surviving mirror copies or data and parity, then writes reconstructed data to a compatible replacement or configured spare. In OpenZFS, replacing a failed device starts a resilver, which processes data known to be out of date. See the OpenZFS pool concepts.
- The array remains at reduced fault tolerance until recovery finishes. Another failure or an unreadable sector may exceed the layout’s remaining ability to reconstruct data. The impact depends on the RAID level and the specific drives and failures, not simply on drive capacity.
- The system returns to its normal redundancy state when rebuilding completes. Check the status tool for the actual NAS, controller, or storage software. OpenZFS reports scan progress and device error counters with
zpool status.
If surviving redundancy cannot reconstruct a file, recovery may require restoring it from a separate backup. OpenZFS states in its scrub and resilver documentation that persistent errors on a file mean the data is gone and should be restored from backup or a snapshot.
How the RAID layout changes the outcome
The following describes common layouts under their normal operating assumptions. Implementations differ, so consult the documentation for the specific controller or storage system.
| Layout | What one failed member means | What recovery can use | Remaining failure margin |
|---|---|---|---|
| RAID 0 | Striped data has no redundant copy; losing a member can make the volume unavailable. | No parity or mirror copy exists in the array to rebuild the missing member. | None. A backup or specialist recovery may be needed. |
| RAID 1 or another mirror | Data can remain available from a surviving mirror copy, if it is readable. | The surviving partner supplies data for reconstruction onto a replacement. | Depends on the number and arrangement of copies; the mirror is exposed while rebuilding. |
| RAID 5 or RAIDZ1 | Single parity can reconstruct one failed member. | Surviving members are read to reconstruct the missing member. Western Digital describes this process in its RAID 5 rebuild overview. | A further failure or uncorrectable read can exceed single-parity protection. |
| RAID 6 or RAIDZ2 | Double parity tolerates more concurrent member loss than single parity, subject to the implementation and failure pattern. | Surviving data and parity are used to reconstruct missing information. | Greater than single parity for supported failure combinations, but not protection against every failure, operator error, or lack of backup. |
Other systems—including dRAID, NAS hybrid RAID, and distributed-parity designs—may have different spare and reconstruction behavior. OpenZFS dRAID, for example, can use a distributed spare and sequential resilver in suitable layouts; that should not be assumed for conventional RAIDZ. See the OpenZFS dRAID documentation.
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Why capacity can lengthen the rebuild
A larger drive can mean more data to reconstruct and therefore more time spent with reduced redundancy. But capacity alone does not determine the finish time. The layout, amount of data that must be processed, drive throughput and health, number of members, controller or software policy, workload, and rebuild priority all matter.
Hewlett Packard Enterprise’s Smart Array SR Gen10 Controller User Guide gives an approximate rate of 15 to 30 seconds per gigabyte for RAID 5/6 rebuilds. That is guidance for the named controller family, not a universal RAID benchmark; HPE says actual time varies with I/O activity, number of drives, rebuild priority, and drive performance. See the HPE Smart Array SR Gen10 user guide.
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Historical examples should not be treated as forecasts for a modern or unspecified array. A Western Digital white paper from circa 2015 modeled a 3 TB mirror rebuild at 19,108 seconds (5.3 hours) assuming 110 MB/s. The same paper modeled 54% greater annual data-loss odds for a 12-drive RAID 5 using 5 TB rather than 3 TB drives, under assumptions including a 40 MB/s sustained transfer rate, a seven-day replacement interval, and a five-year warranty. Those are model outputs under stated inputs, not measured universal rates. The paper is available as Western Digital’s Rebuilding RAID Arrays with Rebuild Assist. IBM also discusses rebuild risk for larger, slower nearline drives, but its analysis does not establish a probability that applies to an unspecified array; see IBM’s RAID-5 and RAID-6 rebuild discussion.
What to do when the array reports a failure
- Identify the member in the storage system’s own interface. Match the indicated bay and serial number before removing anything. Follow the NAS or controller procedure; hot-swap support is not universal.
- Check the array state and the other drives. Determine whether the system reports degraded or faulted, and review other members for read, write, or checksum errors. Do not assume the first failed drive is the only issue.
- Confirm the replacement is compatible. Follow the hardware or NAS compatibility requirements and array geometry. OpenZFS requires a replacement at least as large as the smallest member of the mirror or RAIDZ group being replaced; see its zpool replace documentation.
- Replace the drive and monitor recovery. Watch the controller’s rebuild status or, for OpenZFS, use
zpool statusto check scan progress and per-device READ, WRITE, and CKSUM counters. Investigate nonzero checksum counts rather than simply clearing them; they can point to corruption or another component problem. - Follow the system’s verification procedure after reconstruction. OpenZFS documents that its sequential reconstruction mode does not verify checksums during that rebuild and starts a scrub after it finishes; sequential reconstruction is not supported for RAIDZ. Do not assume another RAID implementation behaves the same way. See OpenZFS scrub and resilver guidance.
- Restore unrecoverable data from a separate backup. Verify the restored files. A spare can start reconstruction sooner, but it is not a backup.
If multiple drives have failed, the volume is faulted, the system reports unrecoverable errors, or irreplaceable data has no verified backup, stop improvising and contact the system vendor or a qualified recovery specialist before taking further action.
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Can you keep using the array during a rebuild?
Often, yes: a redundant array may remain available in degraded mode while it reconstructs. However, performance can be reduced, and the array has less protection against another failure until recovery completes. Avoid unnecessary interruptions and follow the vendor’s guidance for workload and rebuild priority; HPE identifies both I/O activity and priority as factors in rebuild time. Availability is not guaranteed for every RAID level or failure state.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does a rebuild guarantee that all files will survive?
No. A rebuild can restore redundancy only when the remaining data, parity, or mirror copies contain enough readable information to reconstruct the missing member. If an unreadable sector or corruption cannot be corrected using the array’s remaining redundancy, affected files may be unrecoverable. Checksums can help detect corruption, but repair requires a good redundant copy. Restore files that cannot be reconstructed from an independent backup.
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Why RAID still needs a separate backup
RAID helps keep a system running through certain drive failures; it does not protect against every failure pattern, accidental deletion, software or controller problems, or unrecoverable corruption. Keep a separate backup and verify that it can be restored. A replacement drive restores array membership; it cannot substitute for a backup of data the array could not reconstruct.
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