jim,

The backup application doesn't backup videos because those are supposed to be located in your home folder (the big 32GB partition). This partition is completely unaffected during software updates, so there is no point in backing-up 32GB worth of data every time you do a software update.

The back-up feature is more to take care of your software sources (repositories), configurations (wifi passwords) and installed applications (not sure about this one). This is also well documented in the wiki, I'd recommend a bit of reading.

Now, with regards to the loss of parts of your video, I'm afraid there's not much we can do to help. If it's not file corruption and simply the media player not being able to read further than a specific length, and the data is actually missing, then that's that.

If however the data is there, but just in a "hard-to-get" format, then sure, maybe someone would be able to get it back for you.