Some asked this same question but for the 67 so it was closed by the moderator. I am having the exact same problem with my 67i.
Intended result: download the North America Topo Map over wifi, with the GPS itself, using a properly formatted and recogonized SD card with over 28 gb of free space as reported by the GPS itself. Once the intended download destination is set to SD Card, and Download is selected, a pop up message says that additional space is needed (my guess is that is trying to do it using the internal storage and ignoring the SD choice command).
I know I could try Garmin Express and Map Installer (although I have a Mac so it is not always the best experience. I can’t understand why the device itself is supposed to have this funcionality but it not providing the promised outcome. Thanks in advance.