"A failure may be imminent"

In my system log (computer management) I have 45 of these from the past month.

"The driver has detected that device \Device\Harddisk0\DR0 has predicted that it will fail. Immediately back up your data and replace your hard disk drive. A failure may be imminent."

Source is "Disk" and I don't see any other helpful information here.

1) How do I determine which drive is DR0? I have 2 WD JB's in RAID on the HighPoint and a 60GB WD on the reg. IDE.

2) Just what does this mean and how worried should I be?