When I updated my Zumo 660LM with Garmin Express, to the latest maps, the maps, were split between my internal memory and my SD card.

I would like to add the "CN North America NT" map as well. Where do I place the unlocked ".img" files on my SD card, not overwriting the Europe ones.

I believe they can be placed in the root of the Garmin folder. And if you name them gmapsup1.img there should be no interference. You might have to place them in a folder called map.

Unfortunately gmapsup1.img already exists in the Garmin folder on my SD card, I will try making a new folder called Map and put them there to try out

You may want to wait for an answer from someone who is more familiar with your device.