Hi stewbell,
This is a known issue on Retina. On my system happens also trying to move the main screen to the USB screen.
This is described among the known issues:
http://www.displaylink.com/support/ticket.php?id=354
As far as I know there is no known workaround.
It may be possible for 10.9.3 to change this, seeing that it will enable HiDPI modes for external screens. Or at least enable a workaround but I don't have any solution to offer in the short term.
Cheers,
Carlo