The Micro USB cable being used must support both charging, and data transfer. The cable we include new in box with the G80 is about half a meter in length, is all black, and supports both data transfer/charging so the device drive on the G80 can be read by a computer.
Verify the USB end on cable you are using has a GARMIN logo stamped into it. If you do not see the logo on that end of the cable, you are likely using a 3rd party cable that only supports charging, and not data transfer.
Further solutions can be found on the Garmin Support pages linked below which offer in-depth troubleshooting per the specific computer system you are using:
