Unable to find External Disk

Facts:
I got a 500GB WD external disk for my MBP 2.2GHZ/2GB/500GB which with up-dated Snow Leopard and Windows XP pro(Boot camp).

Here's wot happened and my probs:
Last Tuesday I used the external disk with no problem. But on Friday the external disk is unable to be founded by MBP either in Disk Utility or Finder. I tried to switch to Windows XP where I installed MacDrive to read Mac partition under windows. The system says it cannt install the disk coz device unrecognizable. First I suspect it 's the problem with external disk enclosure but when I replaced it with another new one, problem stay still.

So here's some questions, First, is there any instruction I can take to solve the issue, or if the disk unfortunately crashed any possible chance I can rescue the data (My vital working data achieve is inside) using application or else.

Many Thanks !!!!!1

 

I changed the HD to another enclosure newly bought from a local IT store. Both the power cord and USB cable are brand new, defective of cable might not be the problem of this case. But thx anyway

Another update for my problem
I tried to turn the enclosure power off and on the HD sound just as normal even generate heat after a while hook with the enclosure.