Found this thread looking for a USB C cable but saw the discussion so thought I'd chime in.
I have three rinos two 120s and one 130. I'm running Windows 10 Pro and attached them via a usb to serial adapter. Both MapSouce and WebUpdater both found the devices, recognized them and transferred firmware and maps to them.
Researching what I'd need to load map data on them I read that anything newer than Windows 8.1 wouldn't have the drivers for the serial cable. That appears incorrect.
Two issues of note beyond the scope of the post are:
1. I talked with Garmin to ask about what new GPS units are compatible with the 110/120/130 model in regards to proximity location (I use them to keep track of friends hiking) and they said none of the new models work with the old rinos. Looks like I'm going to keep an old machine around just to power on and update maps on my rinos.
2. My two 120s have different software versions. One is 3.90 and the other is 5.90 which is the same as my 130. <shrug>
3. I'm using gpsfiledepot[.]com to download maps which I install and then MapSource sees them. Not suggesting you use it but throwing it the pot with the other information as it's working and allowed me to get info for an upcoming Yellowstone trip.
Cheers,