n addition, dmesg shows this:

Current sda: sense key Medium Error
Additional sense: Unrecovered read error
end_request: I/O error, dev sda, sector 612619550
3w-9xxx: scsi0: ERROR: (0x03:0x0202): Drive ECC error:port=0.
scsi0: ERROR on channel 0, id 0, lun 0, CDB: Read (10) 00 24 83 d5 1f 00 00 03 00
Current sda: sense key Medium Error
Additional sense: Unrecovered read error
end_request: I/O error, dev sda, sector 612619551
3w-9xxx: scsi0: ERROR: (0x03:0x0202): Drive ECC error:port=0.
scsi0: ERROR on channel 0, id 0, lun 0, CDB: Read (10) 00 24 83 d5 20 00 00 02 00
Current sda: sense key Medium Error
Additional sense: Unrecovered read error
end_request: I/O error, dev sda, sector 612619552
3w-9xxx: scsi0: AEN: ERROR (0x04:0x0026): Drive ECC error reported:port=0, unit=0.
3w-9xxx: scsi0: AEN: ERROR (0x04:0x002D): Source drive error occurred:unit=0, port=0.
3w-9xxx: scsi0: AEN: ERROR (0x04:0x0004): Rebuild failed:unit=0.
3w-9xxx: scsi0: AEN: ERROR (0x04:0x0002): Degraded unit:unit=0, port=1.
3w-9xxx: scsi0: AEN: INFO (0x04:0x000B): Rebuild started:unit=0.
3w-9xxx: scsi0: AEN: ERROR (0x04:0x0026): Drive ECC error reported:port=0, unit=0.
3w-9xxx: scsi0: AEN: ERROR (0x04:0x002D): Source drive error occurred:unit=0, port=0.
3w-9xxx: scsi0: AEN: ERROR (0x04:0x0004): Rebuild failed:unit=0.
3w-9xxx: scsi0: AEN: ERROR (0x04:0x0002): Degraded unit:unit=0, port=1.
3w-9xxx: scsi0: AEN: INFO (0x04:0x000B): Rebuild started:unit=0.