Format should be FAT32 for Garmin.
Did you try to create a \Garmin folder on the sdCard and copy there a map file for example?
Does the unit recognize the sdCard then?
In order to update your files (if there is any update available) goto Maps menu and press menu (3 bars).
Press update maps button and will prompt you to insert a usb stick.
Insert a formatted FAT32 stick with a empty Garmin folder on it and the unit will write GarminDevice.xml file in the folder.
Insert the stick in your PC, connect to Garmin Express (or webupdater) and check what happens.
Because of the GarminDevice.xml file, your USB stick will be recognized as your incar unit.