I have found that networking directly to an external USB HD over a LAN was quite a bit slower then accessing the networked computer directly.
So it may end up being quicker to copy (or save) the image to a networked computer & then copy (or move) that image to the attached USB HD on that computer.
Two steps, but it may end up being much quicker (& worth verifying that that is the case for you also - or not).
http://www.pcqanda.com/dc/dcboard.php?az=show_topic&forum=2&topic_id=407212