Garmin Outdoor watches that feature Music storage use MTP (Media Transfer Protocol) mode when connecting to a computer. This USB mode allows for the transfer of media files to the device. Mac operating systems only provide limited native support for MTP devices, which means these devices will not be recognized as external drives when connected to a Mac computer. To transfer music and other audio content to your watch, install the Garmin Express application.
NOTE: Garmin MTP watches can only be recognized by one application at a time. If you happen to have more than one Garmin application open, only one will recognize the watch. If you are having issues with another program detecting your watch, try closing any other Garmin programs that you might have open on the computer.
For information on transferring music to your watch using Garmin Express, see - Installing Music from a Computer to a Garmin Music Watch