Note that some some tower duplicators come with a USB bridge with a special feature: at the flick of a switch, the board goes from pass-through mode to bridge mode, cutting one drive off from the duplicator tower. It can be configured two ways:
1) The bridge can be connected to the controller and an optical drive, allowing you to use one of the drives like an external optical drive. Somewhat useful...
2) The bridge can be connected to the controller and a hard drive, if the duplicator controller supports storing images on a hard drive. This is more useful, as one can manage the images from a computer, not just via the controller.
The only one I've seen, but I believe it's the one that is widely used, can be found via froogle:
It's the "ACARD USB LINK HOST" adapter.
EDIT: also, the "switch" is via a two-pin jumper on the bridge. if the duplicator you have "supports USB", there is probably a pair of pins somewhere on the duplicator board that serves as the signal telling the bridge to engage external-access mode when you request it from the keypad.