I meant also to say that the car is very picky about what USB sticks it will talk to. Brand new Sandisk USB3 didn't work. After borrowing a variety of old USB2 sticks from friends, I found one that works every time, and has a flashing light to indicate read-write activity, so I can be sure the car is actually reading it.
Now discontinued, but there are the odd ones to be found. Extremely slow when extracting the zip file to it (3hrs+), but the car is the limiting factor during map updates.
https://www.nierle.c..._-_USB_2.0.html half-price there, but not from UK.