Just an update on this. I updated my new-to-me 478 last week, and the only thing it would, at first, let me download is "Canada and the border states", similar to the OP here.
(this is on a Mac, FYI).
I called tech support and they had no idea. Their only suggestion was that the 478 didn't have enough internal memory for the new mapset, which is ridiculous.
I kept playing around until I finally stumbled across the solution. If I DESELECTED City Nav on the GPS unit before running the MapUpdater program, then it would allow me to install the whole update without any memory issues, so it must report the GPS as not having enough available memory if the mapset is checked off, which is strange, but apparently true.
Hope this helps someone else who's updating their 478 in the future.