I have the same problem except that they DID play 2 weeks ago so I am dumbfound as to why they cannot be played now. This happened with the previous firmware. The MPG and ISO files played without any problems and then all of a sudden they stopped working.
Now I am not so sure. It may a WD My passport problem. The only thing I can think of is that I added extra files to the My Passport and I had it hooked up via USB for a couple of days…maybe Windows Vista played with files on their?
Sorry I can’t help but maybe gives you other ideas as to what the problem could be.