that sucks man, you think RB would be smart enough to ONLY update changes made and compare to the internal library and NOT delete all your cues. It is simply saying everything on the the usb drive is master and overwrite all data on internal library and that's why it takes so long, instead it should be checking for changes made and update only the tracks changed
this is horrible, I tested this and thats exactly what does happen, RB clearly needs a better synchronization method and programming