5 years ago, this may have been so, but a recent attempt to update my Nuvi 760 to Europe NT 2021.20 disregarded my 8Gb sd card and is forcing me to make a regional choice to load to the device. A support chat with Garmin ended not with a solution, but an offer of a discount if I bought a new device. I drive frequently between Turkey and the UK and need a seamless change from one map region to the next across the whole of Europe. I cant afford roaming data charges to download each region as I need it ...
BTW, I managed to download the whole map update to Basecamp on my laptop ....
Then you simply install the rest of the map to your sd card, follow this FAQ