I had a similar problem and tried a different USB dongle connector. I'm on one of those annoying Macs that only has USB-C ports. It was only charging with my first three-in-one dongle, and on my USB-C to USB adaptor, it recognized my edge 830 right away.
I was having this issue as well. I could connect the Edge to Garmin Connect on my Android phone, but could not get it to connect to my Macbook Pro via a cable and adapter. When I switched to the specific cable provided by Garmin, and the Apple USB-C Digital AV Multiport adapter (the one with USB-C power, HDMI, and a USB connector) the Edge synced up to Garmin Connect on the Macbook immediately. So, seems as though the Edge is sensitive to the cable that is used.
I figured out that my problem was actually my MacBook! I tried syncing on my work computer and it worked fine. So if you happen to have access to another laptop, or if your laptop has multiple USB ports, give that a try.
I was even doing this through an USB C to USB adapter as my MacBook doesn’t have USB ports. It wouldn’t work on any of the ports on my personal Mac. I tried the same adaptor and cable on my work MacBook and it synced right away...
The other thing I noticed was that if you can’t get it to go into sync mode through Garmin Express, unpair your Edge from Bluetooth and your phone and try it again.
I was having this problem too. I have a MacBook with USB-C ports. I have a USB to USB-C adapter. When I used the garmin cable as others have suggested everything worked. The other cables I was using were good (i.e., the device would charge), just that the mac would only mount the device if I used the garmin cable.
