The Mechanism
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.
The Impact on RAID 5
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.
