JTAGged or Non-JTAGged, you're not putting ANY video files on the internal HDD, unless you download them from LIVE and they're digitally signed and then put exactly in the dir they're supposed to be by Microsoft themselves.
If you wanna play MP4, then download the official Media Compatibility Update to your box... and then try putting the videos on a USB stick or USB harddrive maybe? (not sure about MP4 formats playing on xbox360s...)
Otherwise, load the files up in 'Windows Media Center' or 'Windows Media Player', and then stream them over to your networked xbox360. Windows Media Center plays a whole lot of stuff streamed, even MKV files [with Windows7], though it's still a bit buggy sometimes.
But to reiterate: No video files can be put on an xbox 360 internal hard drive to be played. Ever. (unless it's an official trailer or something from M$ themselves on LIVE)
edit: [although this might change sometime in the future for JTAGs if the 'video player update' [that played the test.avi file] is ever completed and released...]
This post has been edited by Firetalion: Jul 26 2011, 04:47 PM