I used Garmin Express on my Window PC without issue, But on Mac the issue of "forever-syncing" was first noticed since OSX Yosemite and now El Capitan.
It always show my 920XT got an update, clicking on "install" , the syncing will take forever without doing anything, in fact, I left my device connected overnight, still showing "syncing". I ended up performing the necessary update via window PC.
I will like Garmin to solve this issue, to me Garmin Express works last during the days of OSX Maverick.