My Nuvi was not being 'seen' by Garmin Express. For two days I have been trying all the solutions offered on the WWW. Then I discovered that it worked perfectly when the laptop charger was disconnected! I fished out the original laptop charger and cable and this, too, works. As soon as I use a non-standard LAPTOP charger, the Nuvi disconnects and shuts down.
My laptop is an ASUS T100TA and is charged via a micro-usb lead. I had a problem on another laptop where a Chinese charger stopped the mouse working! Evidently, some chargers are outputting enough AC ripple to mess up the PC.
I hope this helps someone else.