The Mechanism
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 Impact on RAID 5
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.
