Avago Technologies 3ware SAS 9750-16i4e - Manual (Page 261)

Error and Notification Messages
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001F Unit Operational
Event Type
Information
Cause
Drive insertion caused a unit that was inoperable to become operational again.
Any data that was on that unit will still be there. This message is only sent if
the unit was inoperable for more than 20 seconds. That means that if the hot
swap of a drive occurred within 20 seconds, messages are not generated.
Action
None Required. The unit is available for use.
0021 Downgrade UDMA mode
Event Type
Warning
Cause
The 3ware RAID controller has downgraded the UDMA transfer rate between
the controller and the ATA disk drives. This message only applies to parallel
ATA and certain legacy serial ATA drives.
Background Information
The 3ware RAID controller communicates to the ATA disk drives through the
Ultra DMA (UDMA) protocol. This protocol ensures data integrity across the
ATA cable by appending a Cyclical Redundancy Check (CRC) for all ATA
data that is transferred. If the data becomes corrupted between the drive and
the 3ware RAID controller (because of an intermittent or poor quality cable
connection) the 3ware RAID controller detects this as a UDMA CRC or cable
error. The 3ware RAID controller then retries the failed command three times
at the current UDMA transfer rate. If the error persists, it lowers the UDMA
transfer rate (for example, from UDMA 100 to UDMA 66) and retries another
three times.
Action
Check for possible causes of UDMA CRC errors such as defective or poor
quality interface cables or cable routing problems through electrically noisy
environments (for instance, cables are too close to the power supply). Also
check for cables which are not standard or exceed the ATA specification. A