Try a different USB port.
If your drive still isn't being recognized, turn your PC off, disconnect the data cable, restart the computer, and once the operating system is fully up and running, reconnect the data cable. Give it a minute or two to recognize then you will want to access Disk Management to see if the drive shows up in there. If it does show up in Disk Management, then you most likely have a corrupted or damaged partition and will need to re-partition and reformat the drive for it to be recognized on your PC.
More info here: FAQ: External hard drive no longer shows up in My ... - Western Digital Community
EDIT: Just took a look at your profile and see it says you are using Millineum OS. That could be outdated.
But just in case it isn't, that external drive is only supported in XP and later.
Edited by buddy215, 20 May 2012 - 01:47 PM.