For any RAID type, the most likely cause of the error is a grown defect in the drive. For out-of-synchronization mirrors or parity, the error could be caused by improper shutdown of the unit. This possibility applies to RAID 1, 5, 6, 10, and 50.
When a verify fails, redundant units will automatically resynchronize user data through a background initialization. The initialize will not erase user data, but will recalculate and rewrite user parity data.
If the unit was non-redundant, any data in the error location is lost. (However, the error could be in a part of the drive that did not contain data.) A unit file system check is recommended.
For Mac OS X, you can use the First Aid tab in the Disk Utility—select the disk on the left and then click Verify Disk. If verification encounters problems, you can then use the Repair Disk option on the same screen.
The resynchronization of data that takes place during a background initialization can slow down access to the unit. Once initialization has begun, it cannot be canceled. You can pause it, however, by scheduling it to take place during off-hours. For more information, see
Scheduling Background Tasks. You can also set the initialization process to go slower and use fewer system resources. For more information, see
Setting Background Task Rate. (Initialization occurs at the Rebuild rate.)
