When you have the card inserted, connect to Home and select the external storage when the selection screen appears. That's assuming, of course, that the existing map resides on the card.
Having said that, some report a flaw in the latest Home application such that even if the user DOES select external when Home starts, nevertheless Home tries to put update on internal as well or, in some cases ,internal but NOT the (selected) external!
So, keep an eye on that...................