The 3ware RAID controller was unable to complete a rebuild operation. This error can be caused by drive errors on either the source or the destination of the rebuild. However, because ATA drives can reallocate sectors on write errors, the rebuild failure is most likely caused by the source drive of the rebuild detecting a read error.
The default operation of the 3ware RAID controller is to abort a rebuild if an error is encountered. If you want rebuilds to continue when there is a source error, you can set a unit policy to Continue on Source Error When Rebuilding in 3DM2 or CLI.
The consequence of continuing a rebuild when there is a source error is that there may be corrupt data in your rebuilt unit. In some cases, however, this may be your only alternative for recovering as much data as possible from a unit that has become degraded.
