I do the same with a 64GB SDXC card reformatted to FAT32 used with my nuvis. Most of the maps are in a folder named Spares and I just move them back and forth to the Map folder as required. Much quicker than copying even with a fast-writing Class 10 [UHS-II] card. It's a convenient way to have a vast amount of map data without swamping the unit. Neat trick!