The Mechanism
Modern high-capacity enterprise hard drives have a statistically predictable rate of Unrecoverable Read Errors (UREs)—tiny sectors that naturally degrade and become unreadable over time. In a healthy or moderately degraded RAID 60 array, UREs are easily corrected on the fly by the dual layers of Reed-Solomon parity math.
The Impact on RAID 60
The math changes if a specific sub-array is pushed to its absolute structural limit:
- If a sub-array has one failed drive, it can handle a URE perfectly during a rebuild.
- If a sub-array has two failed drives, it is running with zero remaining redundancy.
If the RAID controller encounters a single pre-existing URE or "silent data corruption" block on any of the remaining healthy drives in that specific leg while attempting a rebuild, it has no remaining parity data to mathematically recalculate the missing sector. The rebuild aborts, the sub-array is marked as failed, and the entire RAID 60 volume drops offline.
