Go to C: drive and open it. The SD card will show up as removeable disk G
Sometimes the SD card will open as removal disk J.
Just check the removable disks type.
Load up the card into the Garmin then plug the unit into the computer via USB. Your computer should then recognise both the internal as well as the SD card. You can load pictures or POIs or music on the Garmin's SD card and they should show up on the GPS.