I'm sorry, I don't have other suggestions. I have no personal experience with such issues, so I couldn't offer more than some vague guesses.
Still I'm pretty sure this has come up before. (more then once)
MusicBee's forum search is not the greatest, but if you use Google, entering something like this in the search bar:
site:getmusicbee.com drag files
I wouldn't be surprised if you would stumble upon a solution.