Work your way through this FAQ, ignore the fact it mentions automobile devices
https://support.garmin.com/en-US/?faq=l0p9W9vsc37NJ3vH2b5pF6&searchType=noProduct
I have a Garmin 645M, When I connect it to my Mac, I'm able to see and add the device in Garmin Express./ However, when I connect it to my Windows PC, Garmin Express doesn't appear to be able to detect or add the device. Although I do see the device in my folder list and can access the files in the device but Garmin Express just not able to find the device. Any Idea?