I have been converting mkv files with Handbrake to mp4 (or mv4) for iTunes so I can use them with my Apple TV. Everything works pretty well apart from a few...... Some of the mp4 files have a strange problem. They play fine on my desktop, but when they are copied to my Qnap NAS, they cannot be played and QuickTime doesn't recognise it as a valid file format. If I then copy the mp4 back to my Desktop from the Qnap, it still doesn't play!!! What can be going wrong when I am just simply copying from my Mac to my NAS? I also copied a couple of movies from youtube, both downloaded as mp4 files and only one of them can be copied to the NAS device and played, the other has the same error!!!