Hi, I also have a similar problem and see greyed out copies of my Tracks in MMA. I don't want to rebuild a new database as the collection is large and my old playlists are in the database (as I understand it). The cause was a need to change my SD card in my Android phone - the old one got 'corrupted...

Try stopping and restarting MMA, it appears to scan as it starts... I had same problem but stop/start worked for me, cheers