Dell POWERVAULT MD3620I Owner's Manual page 217

controller encounters an error while accessing a peer disk, it is unable to
recover the data and affected sectors are added to the unreadable sector log
maintained by the controller. Other conditions under which sectors are added
to the unreadable sector log include:
• A media error is encountered when trying to access a physical disk that is a
member of a non-redundant disk group (RAID 0 or degraded RAID 1,
RAID 5 or RAID 10).
• An error is encountered on source disks during rebuild.
NOTE:
Data on an unreadable sector is no longer accessible.
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