Imported video files in Sage 4 don't play

[ChrisR]

I have quite a large number of MPEG2 files that I have edited over the past couple of years originally captured & viewed by Sage 2, which I've taken & stored as imported videos on an external hard drive. Yesterday I upgraded to Sage 4, and now many of these files will not play. The only files that seem to be affected are files which I transferred to another computer for editing, then transferred back later to be stored away. Most of these were edited exactly the same way, with the same software, yet there is no consistancy with their playback problems: a few files actually play just fine, several play audio but no video (just a black screen), and some don't play at all...to the point of completely locking up the system, and I have to shut it down and reboot. I NEVER had to do this with Sage 2; these files played perfectly fine. Sorry if this was discussed someplace else in another thread, but if it was I can't find it. Hope someone can be of help, I'm a bit flustered at the moment. Thanks!