AEN: DEGRADED_UNIT (unit=0, port=1)
The 3ware driver appears to log any critical error under event id #3 (error).
This particular event seems to indicate that the drive connected to port 1 has failed and needs to be replaced.
If the drive is in a fault tolerant RAID (e.g. RAID 1 or RAID 5), then the logical drive will continue to work, though with degraded performance.