RAID 5 (Striping with distributed parity)

This is the most infamous failure scenario for RAID 5. When one drive fails, the array enters a "Degraded" state. The system continues to work, but it has to calculate missing data on the fly using the parity blocks on the surviving drives.

To restore health, you must insert a new drive, initiating an Array Rebuild. During a rebuild, the controller must read every single bit of data from all surviving drives to recalculate and write the missing data onto the new drive.

Because all drives in an array are typically bought at the same time and endure identical workloads, they age at the same rate. The extreme, sustained stress of a rebuild frequently pushes an already-worn secondary drive over the edge. If a second drive suffers a mechanical failure or stops responding mid-rebuild, the entire RAID 5 array collapses, and all data is lost.

RAID 5 relies heavily on complex mathematical calculations (XOR logic) to process parity data. This is typically handled by a dedicated hardware RAID controller card or system firmware.

If the RAID controller card experiences an electrical surge, overheating, or a physical component failure, the array goes offline. Furthermore, a sudden power outage can result in a "Write Hole." If the system loses power precisely while writing a block of data but before it finishes writing the corresponding parity block, the data and parity become desynchronized. When power returns, the controller may read corrupted structures, leading to file system breakdown.

Enterprise-grade hard drives feature a technology called TLER (Time-Limited Error Recovery). When an enterprise drive hits a bad sector, it spends only a few seconds trying to fix it before reporting the error to the controller, allowing the controller to handle it using RAID parity.

If consumer-grade drives (like standard desktop HDDs) are used in a RAID 5 array, they do not have TLER. A consumer drive may freeze for 30 to 60 seconds trying to recover a bad sector. The RAID controller, expecting a fast response, assumes the drive has completely died and forcibly drops it from the array. If two consumer drives hit bad sectors around the same time, the controller will drop both, instantly killing the RAID 5 array even though the drives are physically intact.

Modern high-capacity hard drives have a statistically predictable rate of Unrecoverable Read Errors (URE)—sectors that degrade naturally over time and become unreadable.

In a healthy RAID 5 array, if a drive hits a bad sector, you won't even notice. The controller immediately reads the parity data from the other drives, reconstructs the missing data, and repairs the sector.

The catastrophe happens when a drive fails, putting the array in a degraded state. Because there is no longer any redundancy, the controller cannot reconstruct data. If the controller encounters a single pre-existing URE or "silent" bad sector on any of the remaining healthy drives while trying to rebuild, the rebuild process aborts completely. The controller marks the array as failed, locking you out of the volume.