You can either let GarminExpress handle where it puts map tiles which is what's recommended, or if you really wish to have certain parts of the States on the device's internal memory and others on the card you can download the mapping data to your computer via the installation options for the new map in GE and then using MapInstall send your preferences to the device without completely filling available space. Finally, disconnect the device and connect the SD card in a reader and you can load the remaining preferred map tiles to the card.