If its USB 2.0 it should be fast enough, I do that all the time. I have tried streaming HD content over USB 2.0 and it worked fine. I usually copy it into my hard drive and disconnect my mobile hard drive before enjoying my home made movies, I just don't like the idea of my mobile hard drive being constantly on, even though I know its just a laptop drive in an external case. I don't know, it is just a habit of mine, I don't think there is any harm in streaming a movie from your mobile hard drive.