Apparently there is a serious problem with the some WD SATA 300 Caviar drives disappearing off the RAID controller for no reason. It appears to be a conflict between the drives' firmware and some RAID controllers. See the Inquirer article here for more info and a link to the fix.
http://www.theinquirer.net/default.aspx?article=37188

We have had a rash of drive problems here as of late. If anyone has seen similar behavior let us know here please.

I keep having a problem with one of my WD 250Gb SATAII Cavier 16Mb drives disappearing occasionally. A reboot fixes the problem. The drive is not formatted as primary partition (blue in Windows Disk Management) but as a simple volume (brown in Disk Managment). I was putting it down to this and is may still be as the 250Gb drive listed is the WD2500YS whereas mine is a WD2500KS.

Thanks for the link though I'll try and see if its necessary for my drives.

edit
Looking at WD site its only for the YS not the KS.

Back to wondering why