I understand that.
So try setting a second SD with the folders

So before you start Garmin Express make sure both SD are installed on your PC.
Make sure you add folders to the blank SD card.
OR one is a memory stick with only those 2 folders.

Then try Garmin Express again.

It should work now that Garmin Express see the other SD card or memory stick.
Hopefully it will then download the maps to the second SD card.

If not I do not know any other way around other than going to another site and downloading unlocked maps