Garmin nuvi 3597 - Express Unable to Recognise Unit
A dumb mistake of mine in the end was causing me frustration as with everything connected to my laptop, the unit was not even visible as a connected USB device.
After rebooting and all the usual Windows things, I finally found through trial and error that you have to plug the USB cable directly into the back of the unit for Windows to see the device, not via the magnetic mount passthrough.