If the device doesn't show up in Mac System Profiler at all or as an unknown device:
The symptoms indicate there is a problem with the drive, how it is connected,
or with the computer used for troubleshooting. The next best step is to change
physical variables, and find a simpler problem than a broken product.
- Change the data cable (USB, FireWire, Ethernet, etc.) A failure in the cable
can cause this issue.
- Change the external power supply or cable. If possible, obtain another power cabling to see if the problem is in
that component. On drives that are powered via USB or Firewire, try using the
included power sharing cable and connect it to another port.
- Change the computer you are using. Computer problems can manifest themselves
in many ways. Even if the computer appears to be working properly, this is a
worthwhile test. Try it on a friend's computer and see if it works fine.
If the tests above don't help, it's possible that the drive was either dropped while running, had a hard drive 'head crash' or the circuitry inside the drive is fried.
Hope this helps!