Hi Mike,
Thanks for the extra details and the update. It does look like an issue with the setup on OS X. In the advanced tab in the setting for the USB to Serial device, if you click on advanced, is it set to null modem? Also make sure the the speed is correct for your device.
If all that checks out and still no improvement, if would be useful to try connecting to another serial device other than the Garmin. Is that possible?
Also, please send your Amazon Order id to us at support@plugable.com
I hope this helps, let me know what you find.
Thanks,
Jerome.