My problem is also along the same lines, I had my 160 gb vault working all of the school year, suddenly it stops recognizing on my computer, sounds as if its on, power chord is working, but it does not connect.
I once did however connect it to my PC after using it on my macbook pro the entire year, and after not recognizing it on my PC it has not worked since, I have a lot of information on here and it would be a huge disaster if these files weren't recovered.
any suggestions would be much appreciated.