Here is the story after a few days of this issue:
1. I am using a Mac Computer with the latest firmware/software installed.
2. I am using the latest Garmin Express app to connect my GPS device (Garmin Montana 750i).
3. I was able to update to the very latest software/firmware onto my device.
4. It "appears" the following maps updated okay (though I am not sure): TopoActive NA, US 2022.20; TopoActive NA, Middle America 2022.20; Garmin Public Land Map V2022.10 ...
however, the City Navigator 2024 does not appear in my device under Maps. When I connect my device to Base Camp, all those maps appear PLUS City Navigator 2024, in fact it reads it
successfully and even displays the map on my computer. So...it IS loaded but apparently it is locked for some strange reason. As a result, whenever I turn on my device, the error message
"Cannot unlock Maps". A similar message appears (with just different wording) when I insert older Garmin City Navigator for U.S., Asia, and Europe. None of those work, yet they worked
in my older Garmin Montana. Keep in mind, these older maps on my MicroSD card can be used in multiple devices.
5. I have a non-Garmin Map of Japan (Manufactured by a Japanese company called "UpUpDown" on MicroSD and it displays just fine. It even appears in the list of maps and I even have
the option to disable it if I want to. This confirms that my MicroSD slot in my Garmin unit is NOT defective and the unit does not appear to be defective in any way.
6. Is there any way to uninstall the map and copy it on to a MicroSD card (32GB max allowed, I believe) and just use it that way?
7. What else can I do to get rid of the error message "Cannot unlock maps".
Your help is appreciated. Thank you.