Thanks. Yes, I tried the .iso and also the renamed .dvd format, but neither format worked. Although, from reading the forum, I don't think I should even need to convert to iso or .dvd - the VIDEO_TS should work - is that correct?
Yes, this is a USB external HDD. I am not at home at the moment but I'll give that a go when I get back. However, I still don't think that is the problem either, because the .dvd files i created using linuxmce are also stored on the external drive, and these are working fine.
Thanks again for your input guys, if you can think of anything else that might work, let me know!