Yes you can manage the maps on an sd card, I have topo maps on mine. Just download the map you require using mapsource to the SD car instead of the gps (The card will need to be in the gps) DO NOT CHANGE THE NAME OF THE FILE AND DIRECTORY THAT MAPSOURCE CREATES!!!
The Garmin Nuvi 760 and higher definitley capable of reading maps and particularly POIs from an external drive (SD-mirco card). Caution is warranted however if using a high capacity card. These cards require the use of a HC card reader to be able to copy POIs and maps to. Once on the card, you can access the files like any other drive.