The Mechanism
Over years of continuous use, magnetic storage media or solid-state cells can naturally degrade, resulting in Unrecoverable Read Errors (UREs)—bad sectors that the drive can no longer read.
The Impact on RAID 10
If a drive fails in Mirrored Set A, the array keeps running. However, if the surviving twin drive in Set A happens to have a "silent" bad sector or encounters a URE precisely during the rebuild process, the controller cannot complete the clone. Because there is no secondary parity layer across the other sets to calculate the missing piece, the rebuild will stall, abort, or mark the entire volume as failed.
