I have a Buffalo HD-PX320U2 external Hard Drive which I bought in Kuala Lumpur a couple of years ago.
I had it plugged into my laptop and it was working fine. My laptop then crashed and I rebooted. The computer then wouldn't recognise the HD. So I unplugged the HD without ejecting (as the laptop couldn't see it)
I now can't get any computer to recognise it. I have tried it on two separate latops running W7, a desktop with W7 and a latop running XP.
I have also tried using a different usb cable but that hasn't made any difference.
What happens when I plug it in is the blue light on the HD unit flashes for a second and then goes out. And that's it.
based on your troubleshooting it is definitely a problem with your external drive. You can try to remove the disk from the external enclosure and install it inside a desktop computer to see if it works (or put it in a different external hard disk enclosure). This will at least let you know if the drive itself is okay or not.