Whichever map you want, Europe or North America has to be obtained LEGALLY from tomtom.com.
Assuming that to be the case, the usual method, is to insert the card into the unit, connect the unit to the computer, start Home, select internal or external storage, then go to Add Maps, purchase the appropriate map and have Home dl and install it to the selected storage area (internal or card).
If you already have a map stored on internal, then you don't need or want Home involved. Connect the unit to the computer with the card installed. Use Windows explorer to move the specific map folder (and contents) from internal to the card.
But ..... But .....
That will only work if you have a Go 7x or 9x model that sees internal and external storage in conjunction with each other.
If you just want to make a backup of your map onto a card, then use Explorer to COPY the map folder from internal to the card.
Why do I get a feeling you aren't telling us the complete story here ??