So I have been meaning to look into this for a while. Most of my computers have no issue but my wife's computer has iTunes with all of our music in that folder. And to avoid messing up configuration of all of that and playlists and whatever, I wanted to sync that music folder but not move it into the main dropbox folder. This morning I was fucking around with it and realized with windows 10 (and probably 8), you can create a symlink in the dropbox folder to the external folder.
HOWEVER, Windows, being the fickle bitch that she is, doesn't sync changes immediately. So if you put something in that external folder, the dropbox mechanism won't pick it up until you stop/resume the sync. Once I realized that, it worked perfectly. So now I have my solution. Our music doesn't change that often and my wife restarts her computer often enough to give me a comfort level that everything will remain backed up (we don't buy new music that often).
Anyways, just thought I'd share in case anyone else has that situation.