RAID is often treated as a form of data protection. In practice it is a form of availability engineering — it keeps systems running despite individual drive failures, but it does not protect against many of the events that cause real data loss. A significant proportion of the RAID cases we handle begin with a rebuild that went wrong rather than with the original drive failure itself.
The first common error is replacing a failed drive in a degraded array without first verifying the health of the remaining drives. In RAID 5 in particular, the rebuild process reads every sector of every surviving member drive. If any one of those drives has undetected read errors, the rebuild will fail partway through, and the array will typically drop further members in the process.
The second common error is forcing a drive back online after a brief disconnection. Most controllers will mark a drive as foreign or failed after a timeout, and the correct procedure is to investigate before re-including it. Forcing the drive back into the array can cause the controller to use stale parity data, silently corrupting the file system from the inside.
The third is changing controllers mid-failure. A RAID array is specific to the controller firmware that wrote it. Moving disks to a different model or even a significantly different firmware revision during recovery can make the original metadata unreadable, and from that point onward the array must be reconstructed from first principles rather than simply reassembled.
Where an array has already degraded, our standing recommendation is simple: power the system down, preserve the physical drive order, and consult before acting further. An incident that begins as a single drive fault can almost always be resolved. One that has been through a failed rebuild is a materially harder case.
