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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteWhen a drive fails in a redundant RAID array, the system usually marks the array degraded and continues serving data from the remaining drives while it rebuilds onto a compatible replacement or spare. The array has less protection until recovery finishes: another failure or an unreadable sector may exceed what its RAID level can reconstruct. The precise outcome depends on the RAID layout, implementation, and condition of the surviving drives.
What happens after a drive fails
- The system detects a missing or failed member. A redundant array may keep operating in a degraded state; if it no longer has enough copies or parity to function, it may become faulted. OpenZFS, for example, distinguishes online, degraded, and faulted pool states in its pool-state documentation.
- Missing data is reconstructed if the layout allows it. A replacement drive or configured spare receives data rebuilt from surviving mirror copies or parity. In OpenZFS, replacing a failed member starts a resilver, which processes data known to be out of date; see OpenZFS replacement guidance.
- The array remains at reduced fault tolerance while recovery runs. A further failure or an unrecoverable read can exceed the remaining redundancy, leaving some files or the whole volume unavailable.
- Normal redundancy returns only when recovery completes. Use the status tool for the actual NAS, controller, or storage software. For OpenZFS,
zpool statusreports scan progress and per-device READ, WRITE, and CKSUM counters. - Unrecoverable files must be restored from backup. OpenZFS warns that persistent errors on a file mean the data is gone and must be restored from a backup or snapshot.
What the RAID level can recover
| Layout | What one failed member means | Rebuild and remaining risk |
|---|---|---|
| RAID 0 | No member failure is tolerated; losing a drive can make the volume unavailable. | There is no mirror or parity from which to rebuild. Recovery generally requires a backup or specialist data recovery. |
| RAID 1 or another mirror | A surviving mirror copy can supply data, provided it remains readable. | The surviving partner is read to reconstruct the replacement. Another failure in that mirror before recovery can leave data unavailable. |
| RAID 5 or RAIDZ1 | Single parity can reconstruct one failed member under normal assumptions. | While a member is missing, a further failure or an unrecoverable read can exceed the remaining protection. Western Digital describes RAID 5 recovery as reading surviving members to reconstruct the failed member. |
| RAID 6 or RAIDZ2 | Double parity tolerates more concurrent member loss than single parity, within the implementation’s limits. | It does not cover every combination of failures, corruption, operator mistakes, or controller problems, and it is not a substitute for backup. |
These are broad layout properties, not a guarantee for every controller or NAS. ZFS mirrors, RAIDZ, dRAID, hardware RAID, hybrid NAS layouts, and distributed-parity systems can differ in recovery behavior. For example, OpenZFS dRAID supports distributed spares and sequential resilvering in suitable layouts; that behavior should not be assumed for conventional RAIDZ.
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Why a large drive can mean a longer rebuild
Reconstruction can take longer as the amount of data to process increases, but capacity alone does not determine the finish time. Layout, drive throughput and health, controller or software policy, array size, workload, and rebuild priority all affect duration.
Hewlett Packard Enterprise’s Smart Array SR Gen10 Controller User Guide gives an approximate RAID 5/6 rebuild rate of 15 to 30 seconds per gigabyte for that controller family; its publication year is not stated in the search result. HPE says actual time depends on I/O activity, the number of drives, rebuild priority, and drive performance. This is a product-specific guideline, not a universal estimate. See the HPE Smart Array SR Gen10 Controller User Guide.
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Western Digital’s circa-2015 white paper gives historical modeled examples, not general forecasts: a 3 TB mirror rebuild was modeled at 19,108 seconds (5.3 hours) assuming 110 MB/s; a separate comparison modeled 54% greater annual data-loss odds for a 12-drive RAID 5 using 5 TB rather than 3 TB drives. That comparison assumed a particular rebuild calculator, 40 MB/s sustained transfer, a 12-drive array including parity and hot spare, a five-year warranty, and a seven-day replacement interval. Those assumptions and the paper’s age make the figures unsuitable as a prediction for an unspecified modern array. IBM’s discussion of nearline-drive rebuild risks likewise depends on configuration and model inputs; it does not establish a universal failure probability. See Western Digital’s RAID Rebuild Assist white paper and IBM’s RAID-5 and RAID-6 rebuild discussion.
What to do when a member fails
- Identify the correct drive. Check the storage system’s management interface and confirm the bay and serial number before removal. Follow the exact enclosure or controller procedure; not every system supports hot swapping.
- Check the whole array’s health. Confirm whether it is degraded or faulted and inspect other members for errors. Do not assume the first reported failed drive is the only problem.
- Check replacement compatibility and size. Follow the controller or NAS compatibility requirements and array geometry. OpenZFS requires a replacement at least as large as the smallest member of that mirror or RAIDZ group.
- Replace the drive and monitor recovery. Keep the system’s status page or command-line tool open until rebuilding or resilvering is complete. In OpenZFS, use
zpool statusto check progress and READ, WRITE, and CKSUM counters. A nonzero checksum count can indicate corruption or another component problem; investigate rather than simply clearing the counter. - Allow recovery to finish and verify the result. Avoid unnecessary interruptions and follow the system vendor’s guidance on workload and rebuild priority. OpenZFS sequential reconstruction does not verify checksums during that rebuild mode and starts a scrub afterward; sequential reconstruction is not supported for RAIDZ. Perform any verification or scrub required by your storage system.
- Restore files that cannot be reconstructed. Recover them from a separate, verified backup. 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 the data is irreplaceable and you lack a verified backup, stop improvising and contact the system vendor or a qualified recovery specialist.
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Why RAID still needs a separate backup
RAID redundancy helps a system keep operating through certain drive failures; it does not protect against every failure pattern, corruption, operator error, or a file the array cannot reconstruct. Keep a separate backup and periodically verify that files can be restored. A replacement hard drive restores array capacity only when it is compatible with the specific controller or NAS; it cannot recover data that the remaining redundancy has already lost.
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