One of the bain's of my 3d printing existence is when Makerware or replicator g does not connect to the replicator dual. So then you just replug it into a different usb port. Then you restart the Makerbot, still nothing. After that you, wondering whats going on, look back at you computer and realize that you still have makerware open if your trying to use rep g or vica-verca so the one that your not trying to use is "taking up" the usb port. To fix this problem just close both programs completely then just open the one that you want. Your done.