My 360 no longer plays video files that it used to.

Allo;

on the 360: My Video Apps -> System Video Player -> PC -> media file == does not play

on the 360: My Video Apps -> System Video Player -> Portable Device -> media file == plays just fine

(the "Portable Device" is simply a USB stick)

so it's the same player on the 360, same file, but two different "locations". Should play them both it seems... no?

I have the same problem. The video apps don't update their respective libraries properly or in some cases at all it seems. What I do is explore the drive using a file manager app (I use File Explorer) which allows me to see all the files in each directory
including the root directory. Then I just select the file thereby opening it which then prompts you to select the app you want to play the video from a list that only contains the apps that are able to play that type of video file. The app takes a few seconds
to load on my OneX, probably longer on the other consoles, and video starts playing. Oddly even after playing it, it still won't show in that player's library.

No, there's no way to transfer video files to the Xbox 360 hard drive. The best way to play your video files is by putting them onto a USB drive and playing them from the built in video player.

The Xbox does not play VOB files. support.xbox.com/.../audio-video-playback

USB isn't supported at launch but should be patched in at a later date from what I've read. It sucks, I know.