Another twist : -
Today I made a route in BC, but when I plugged the Zumo in to transfer it, yet again my computer, or BC, did not recognise it.
After a lot of swearing and trying every possible variation of turning things on and off, and timings of plugging things in, I cracked the code.
For the mental health of anyone using a Mac operating system of Catalina, Big Sur or later and who are having problems, follow the following procedure : -
1) follow the directions here
Garmin Express not Detecting Devices on Mac Catalina 10.15 and Big Sur 11.0 and Newer
2) get a new cable
3) uninstall then reinstall Garmin Express
Then, when you want to use BC and Zumo, use the following procedure : -
1) have your laptop/computer turned on, and Zumo turned off
2) connect the cable to the Zumo and the laptop/computer
3) then, and only then, open Basecamp and wait until the programme recognises the Zumo. On my laptop this took about 4 minutes.
Trying any other variation resulted in BC not recognising that the Zumo was connected.
Why the hell Garmin makes life so difficult is beyond my understanding.