3DM 2 Online Help

0021 Downgrade UDMA mode
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 (e.g., an intermittent 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 (e.g., from UDMA 100 to UDMA 66) and retries another three times. This message is sent to you when the 3ware RAID controller lowers the UDMA transfer rate.
Possible causes of UDMA CRC errors are bad interface cables or cable routing problems through electrically noisy environments (e.g., cables are too close to the power supply).

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