I have a nuvi 2599 with SD card. If I use Windows 10 and Garmin Express it finds and uses the SD card in the Nuvi without any problem. If I use Garmin Express on a Mac then it doesn't find the SD card, even though it mounts OK on the Mac desktop. IT seems that teh combination of Garmin Express 2.3.7 (71), Mac OS El Capitan 10.11.5 and an SD card doesn't work.
The current version of Garmin Express that should be running on El Capitan (10.11.6) is version 4.1.23.0. If you are not running this version, you should uninstall the 2x version you have and install the current version.