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This is usually an issue with the Firmware, so I would first like you to do this please.
Remove the charger from the Garmin and hold the power button to off for 60 seconds.
Release, then connect the charger and try to turn on the Garmin and connect it.
Let me know the results please
ok, can you connect it to a different USB port on the computer and tell me please if it is then detected?
ok, disconnect it
Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
Turn on your Mac.
Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
Hold these keys until the computer restarts and you hear the startup sound for a second time.
Release the keys.
When the mac starts, connect it and tell me if it now is detected please.
its the one with OPT on it
ok, you have a windows keyboard, yes it is Alt in your situation.
The Command (⌘) button., do you have this?
ok, let me know the results
Do you have another computer there?
ok, connect it to that computer and tell me if it is detected please
We need to reset the Garmin. This will remove all data and return it to new.
To do this
Once done, set it up and tell me if it then connects please
its either the cable or the units port has faulted.
Can you try with a different cable please?