How to Rebuild a Failed RAID Without Losing Data

RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drives to offer greater storage capacity, speed, and fault tolerance. However, like all storage systems, RAID arrays can still fail. Rebuilding a failed RAID array properly is crucial to restoring redundancy and preventing permanent data loss. This guide will walk you through the steps needed to successfully rebuild a failed RAID array without losing your precious data.

Understanding RAID Levels

The first step is to identify your RAID level, as rebuild methods differ between configurations. RAID 0 stripes data across disks for performance, but offers no redundancy. RAID 1 mirrors disks for 100% redundancy. RAID 5 stripes data and parity information across drives, allowing one disk failure. RAID 6 offers dual parity to withstand two disk failures. Know your current RAID level before attempting a rebuild.

Identifying a Failed RAID

A failed disk is usually obvious from hardware or software RAID monitoring tools showing the faulted drive and degraded array status. Common signs also include inaccessible data and reduced performance. Determine the failed drives causing the degraded array. Failures can result from many issues like disk errors, controller failure, overheating, etc.

Preparing for Rebuild

Before starting, backup critical data as an extra precaution. Replace any physically damaged drives with new high-quality drives of appropriate specifications. Verify drive integrity with manufacturer tools before insertion. Gather necessary cables, drivers, utilities, and documentation. For hardware RAID, ensure the RAID controller and management software are updated.

Rebuilding the Array

The rebuild process varies by configuration:

RAID 1: The replacing drive is mirrored to the functional drive. Data is recopied over while maintaining full redundancy.

RAID 5: Parity data is recalculated and spread across all drives along with the user data. Full redundancy resumes after completing.

RAID 6: Both parity drives are recomputed using all available data drives. Operation remains fault tolerant.

Refer to the DiskInternals RAID Recovery documentation of your RAID management software or controller for specific recovery instructions. One of the most apparent advantages of free software raid is the cost savings, as users are not required to invest in expensive hardware RAID controllers. Monitor rebuild progress, which can take hours or days depending on array size.

Verifying a Successful Rebuild

When finished, verify that the RAID reports full data redundancy and fault tolerance again. Run data integrity checks like chkdsk in Windows or fsck in Linux to confirm no filesystem corruption. Perform read/write disk benchmark testing under heavy load to stress test stability.

Data Recovery

If the rebuild fails and data loss occurs, quickly power down and turn to professional data recovery services to attempt restoring data from the failed disks. As a last resort, specialized tools may reconstruct files by carving residual data from the drives. However, prevention via backups remains the best way to protect important data.

Maintaining Your RAID

Routine maintenance is crucial for preventing avoidable RAID failures:

  • Monitor disk health statistics for early failure warnings.
  • Replace disks at the first sign of issues before faults occur.
  • Keep firmware, drivers, and management software updated.
  • Maintain complete and redundant data backups.


Following these best practices for proactive RAID care reduces the likelihood of failure and need for stressful, time-consuming rebuilds. But should a disk fail, this guide has provided you the essential steps to successfully rebuilding your RAID and restoring redundancy without permanent data loss. With the right preparation and procedures, you can rise resiliently from the ashes of a crashed RAID.