I have a 660 and I solved the problem just by not selecting all parts of the map before tranferring the map from Mapsource. It is highly unlikely that one needs all areas at the same time. Should this be the case, use two SD-cards with overlapping maps and switch cards when the need arises.
I am not sure what would happen if you put two different parts with one part in internal memory and the other part in the SD-card. Transferring large maps to internal memory takes so looong time, usb1 unforunately.