RAID Rebuild Failed: What to Do Next

In parity-based arrays (RAID 5, RAID 6), a rebuild reads every sector of every surviving drive to recalculate the missing data onto a replacement. Mirrored arrays (RAID 1, RAID 10) read only the surviving mirror partner, making their rebuilds faster and less stressful. In either case, three categories of failure account for nearly all rebuild aborts: latent sector errors, second mechanical failures, and controller-level errors.

• 1.Latent sector errors (UREs): Sectors that became unreadable at some point but were never accessed, so the error went undetected. The rebuild forces a full sequential read that surfaces every latent error. On high-capacity drives, the probability of hitting at least one URE increases with total bytes read.
• 2.Second drive failure: Drives purchased together accumulate similar wear. If one has failed, the remaining drives have experienced identical power-on hours and thermal cycles. The sustained sequential I/O of a rebuild accelerates failure in drives already near the end of their service life.
• 3.Drive and controller timeout mismatch: RAID arrays depend on drives responding within strict time limits. Enterprise and NAS drives set their internal error-recovery timeout (ERC/TLER) to approximately 7 seconds, ensuring they either return data or report failure quickly. The RAID controller imposes its own command timeout on top of this, typically 8 to 20 seconds depending on the vendor. Consumer desktop drives, which lack ERC configuration, may spend 30 seconds to over 2 minutes retrying bad sectors internally. This mismatch is the root cause of 'phantom' drive drops: the drive is still working, but the controller's patience runs out first, and it marks the drive as failed.

For RAID 5 rebuild failures specifically, the risk is highest because RAID 5 has zero remaining fault tolerance once degraded. RAID 6 and RAID 10 have additional margin, but the same physical failure mechanisms apply.

Before deciding on a course of action, gather information about the array state without modifying anything on disk. The goal is to determine whether the failure was transient (cable, timeout) or physical (media degradation, mechanical fault).

• 1.Record the controller error. The exact message narrows the diagnosis. "Media error on PD 2 at LBA X" points to a specific drive and sector. "PD 3 not responding" suggests a mechanical or connection failure. Note the rebuild percentage at failure.
• 2.Check SMART data on all drives. Use smartctl -a /dev/sdX (Linux) or the controller's management utility. Key attributes: Reallocated_Sector_Ct (sectors already moved to spare areas), Current_Pending_Sector (sectors queued for reallocation), and Offline_Uncorrectable (sectors that failed offline scan). Non-zero values on any of these indicate degraded media.
• 3.Document the RAID configuration. Record the controller model, firmware version, RAID level, stripe size, write policy (write-back vs write-through), and number of drives. This information is required for offline reconstruction if controller metadata is damaged.
• 4.Label every drive. Mark each drive with its physical slot number using tape or a marker on the drive itself (not just the tray). If drives are removed for imaging, the slot mapping must be preserved.

For detailed guidance on reading controller logs across Dell PERC, HP SmartArray, LSI MegaRAID, and Linux mdadm, see the degraded RAID troubleshooting guide.

Some rebuild failure scenarios leave the array in a state that cannot be safely resolved with standard administrator tools.

• 1.Multiple physical drive failures. If two or more drives have mechanical problems (clicking, not spinning, SMART reporting thousands of reallocated sectors), the drives need to be imaged with hardware that can manage bad sectors, weak heads, and firmware faults at a level ddrescue cannot.
• 2.Partial rebuild corrupted parity data. If the controller wrote partial parity updates before the rebuild failed, the array cannot be reassembled using either the pre-rebuild or post-rebuild state without analyzing which stripes were modified. This requires forensic RAID reconstruction that compares parity states across drives.
• 3.Controller metadata is damaged or missing. If the controller BIOS no longer shows the virtual disk, or shows it as "Foreign" or "Missing," the metadata defining stripe size, drive order, and parity rotation may be corrupted. Reconstruction requires scanning the raw drives to detect RAID parameters from data patterns.
• 4.Post-failure operations already modified the drives. If someone has run force-online, fsck, or reinitialized the virtual disk, the on-disk state has been modified. Recovery is still possible in many cases, but the window narrows with each modification.

For RAID data recovery involving physical drive faults, we image each drive with PC-3000 and DeepSpar Disk Imager through write-blocked connections, then reconstruct the array offline in software. The original drives are never written to. For RAID 5 arrays with partial rebuild corruption, we analyze stripe-level parity to determine which sections use pre-rebuild vs post-rebuild data.