the external drive on different USB ports? (I am ASSUMING a USB ecternal as you havent' told us but the same suggestion applies to SCSI, E-SATA, or Firewire externals)
While the external is NOT attached take a look at the Applications and System logs with your Event Viewer and see if any problems are noted.
Follow Bob's advice and make that Ubuntu Live CD or download this smaller purpose positioned Live Rescue CD which is only 175 meg and see if you can boot the CD with that external attached then use Midnight Commander File manager to see if you can see the external drive. This can help determine if the problem is hardware or software.
Try the external on a powered USB hub.
I hadn't turned on my external HD for some weeks, but now when I do my pc freezes up, except that I can move the curser, which will move about the screen. I cannot open, or close any programs, and Control/Alt/Delete does not open the usual list of activities. When I try to close a program I get the message that it is "unresponsive". When this happened repeatedly I assumed there was a fault with the HD and spent £50 on a new one, only to find the pc behaves in exactly the same way with the new HD. I have run Chkdsk and no errors were found. What can I do next, please? Running XP Home.