This has happened a number of times to me  - I have hundreds of home videos that in the past I had tagged with people tags etc.  These live in a folder on my D: drive instead of the default C: drive.

When photo gallery updates (and it happened recently also though I didnt think photo gallery needed an update this time) it loses the folders on the D drive from the library.  I then have to add the folders to the library again and it rebuilds the library - this isnt to bad for the photos because the metadata is stored in the photos and can therefore be retrieved - however for the videos it is stored elsewhere and is now lost.

It would be really great if photo gallery did not dump this information (ie the Library locations and hence all video metadata) whenever it updates.