How to Fix a Degraded RAID Array Without Losing Data
2026-10-14 · LC Repair
Your RAID array is degraded. The RAID controller or software is reporting that one or more drives in the array have failed or dropped out, and the array is running in a reduced state. A degraded RAID array still works — your data is accessible and your system continues to operate — but you have lost the redundancy that protects against data loss. If another drive fails before the array is rebuilt, you lose data. This makes a degraded RAID a time-sensitive situation that requires careful action rather than panic.
What degraded means for each RAID level
RAID 1 (mirror): One of the two mirrored drives has failed. The remaining drive contains a complete copy of all data. The array continues to function normally at full performance, but there is no backup — if the surviving drive fails, all data is lost
RAID 5 (striping with parity): One drive in the array has failed. Data that was on the failed drive can be reconstructed from the parity information distributed across the remaining drives. Performance is reduced because every read now requires parity calculations. If a second drive fails, the entire array is lost — this is the critical risk during a RAID 5 rebuild
RAID 6 (double parity): One or two drives have failed. RAID 6 tolerates two simultaneous drive failures. If one drive is down, the array is degraded but still has one level of redundancy. If two drives are down, the array is critically degraded — no further failures can be tolerated
RAID 10 (striped mirrors): One drive in a mirror pair has failed. The surviving mirror in that pair continues providing data. The array can tolerate additional failures as long as both drives in a single mirror pair do not fail simultaneously
Immediate steps when RAID degrades
Do not ignore the warning. A degraded RAID array is the single most important hardware alert your system can produce. Every hour the array runs degraded is an hour where a second failure means data loss. Treat this as urgent
Back up immediately. Before doing anything else, back up all critical data from the array to an external drive or another system. The array is still accessible — use this window to create a safety copy. If something goes wrong during the rebuild process, this backup is your insurance
Identify the failed drive. Check the RAID management utility to determine which specific drive failed. Hardware RAID controllers have a management interface accessible during boot (usually Ctrl+I for Intel RST, or a dedicated key shown on the boot screen). Software RAID on Windows shows status in Disk Management or Storage Spaces. Linux shows status in the mdadm detail or through the controller utility. Note the drive serial number and physical location
Check system logs. Windows Event Viewer (System log) and Linux system logs (dmesg, syslog) contain entries about the drive failure. Look for SMART warnings, I/O errors, or timeout messages preceding the failure. This information helps determine if the failure was sudden (indicating total drive death) or gradual (indicating deteriorating health with possible recovery)
Determine the cause of failure
Drive death: The drive has completely failed and is not detected by the system at all. No spin-up sound, no activity light, not visible in BIOS. This requires a replacement drive
SMART errors: The drive is still detected but SMART monitoring shows critical errors — reallocated sectors, pending sectors, or uncorrectable errors above threshold. The drive is failing and will continue to degrade. Replace it before it dies completely
Cable or connection issue: The drive is fine but the SATA cable, power connection, or backplane connector is loose or faulty. Check physical connections. If reseating the cable brings the drive back, replace the cable to prevent recurrence. The RAID controller may require a manual rebuild even after the connection is restored
Controller issue: Rarely, the RAID controller itself develops a problem with one port. Test the drive in a different port or as a standalone drive to rule out controller port failure
Rebuild the array
Replace the failed drive: Install a replacement drive of equal or greater capacity. Use the same model if possible for consistent performance across the array. The replacement drive must be at least as large as the failed drive — a smaller drive cannot be used even if the RAID does not use the full capacity of each disk
Initiate the rebuild: Through the RAID management utility, assign the new drive as a replacement (hot spare or rebuild target). The controller begins rebuilding — copying data and recalculating parity to restore the array to its full redundant state
Rebuild time: Rebuilding a RAID array takes hours to days depending on array size, drive speed, and system load. A 4 TB RAID 5 array with standard hard drives typically takes 12 to 24 hours to rebuild. SSDs rebuild significantly faster. During the rebuild, the array remains accessible but performance is reduced
Do not interrupt the rebuild: A power loss or system crash during a rebuild can corrupt the array or force the rebuild to restart from the beginning. Use a UPS (uninterruptible power supply) during the rebuild process. Avoid restarting the system unless absolutely necessary
Reduce load during rebuild: The rebuild process puts significant stress on the surviving drives because every sector must be read. Minimize system activity during the rebuild — this is not the time to run backups, defragmentation, or intensive workloads on the array. Reducing load also speeds up the rebuild
RAID 5 rebuild risk
RAID 5 rebuilds are the most dangerous because every sector of every surviving drive must be read during the rebuild. If any surviving drive has an unreadable sector (a latent error that was previously unnoticed), the rebuild fails and the array is lost. This is called an Unrecoverable Read Error (URE)
The probability of hitting a URE increases with drive size. On large arrays (drives over 2 TB), the statistical likelihood of a URE during a RAID 5 rebuild is significant. This is why RAID 6 or RAID 10 is recommended over RAID 5 for large drives — they tolerate the additional failure that a URE during rebuild represents
If you have a large RAID 5 array with older drives, consider the rebuild a high-risk operation and ensure your backup is complete before starting
Hot spares
A hot spare is an unused drive installed in the system and configured as a standby replacement. When a drive in the array fails, the RAID controller automatically starts rebuilding onto the hot spare without human intervention. This minimizes the time the array spends in a degraded state
If your RAID controller supports hot spares and you have a bay available, adding one is the most effective way to reduce the risk window. The rebuild starts within minutes of the failure rather than waiting hours or days for someone to notice the alert and install a replacement
Prevent future degradation
Monitor SMART data: Use monitoring software (CrystalDiskInfo on Windows, smartmontools on Linux) to watch drive health continuously. SMART warnings about reallocated sectors or pending sectors indicate a drive is beginning to fail — replace it proactively before it degrades the array
Replace drives proactively: Drives in a RAID array are the same age and have experienced the same workload. When one fails from wear, the others are at similar risk. Consider replacing all drives in the array within a reasonable timeframe after a failure rather than just the failed one. Stagger replacements to avoid running on all new untested drives simultaneously
Use a UPS: Sudden power loss during write operations causes the most severe RAID corruption. A UPS protects against this by providing enough power for a clean shutdown during outages
Test backups regularly: RAID is not a backup — it protects against drive failure but not against file deletion, corruption, ransomware, or controller failure. Maintain separate backups of critical data and test restoring from them periodically
RAID repair help in Columbus and Delaware
If your RAID array is degraded and you need help identifying the failed drive, safely rebuilding the array, or recovering data from a failed array, Luxurious Computers can diagnose the failure, perform the rebuild with proper precautions, and configure monitoring to catch future drive issues before they cause data loss.