The Mechanism
Large-capacity enterprise mechanical hard drives suffer from statistically predictable Unrecoverable Read Errors (UREs)—isolated, microscopic sectors that naturally degrade over time and become unreadable.
The Impact on RAID 50
If all sub-arrays are healthy, hitting a URE is a minor issue; the RAID 5 logic uses its parity blocks to instantly reconstruct the missing data. However, if one sub-array is running in a degraded state (missing a drive) and encounters a single URE on any of its remaining drives during a rebuild or heavy read operation, it lacks the necessary mathematical parity to resolve it. The controller will immediately abort the rebuild, mark the sub-array as failed, and offline the entire RAID 50 volume.
