I was thinking something else in regards to the internal/external...
Lets try something else. If you don't already, install Garmin Basecamp. If you have a mac you need Basecamp and I believe map install as well, a seperate download. Now download the "Mac" version of CA-CentralProvTopo. Unzip and you should be left with a folder ending in .gmapi If you have windows click to open that folder, keep opening until you see several files and folders including product1 and a couple of others. Back up one level and you should see CA-CentralProvTopo.gmap (no i) folder. Copy this folder to C:\ProgramData\Garmin\Maps you might need to unhide and create those folders, as they are spelled. If it is a mac, word on the street is that after unzipping you will be left with an icon, if basecamp and mapinstall are installed correctly, click that an it should install the map. Regardless if it is mac or win when you open basecamp you should see CA-CentralProvTopo in the maps drop down menu, now click it and let the tiles load.
At this point plug in your Montana. Give it a minute, then towards the bottom of the map menu you should see install maps, open that and your gps should show, highlight your map, click continue, select as few or as many tiles as you'd line and follow the prompts to send the map to your gps if everything is going right it may take 5-10 minutes to install the map. After it is done, power cycle gps, enable CA-Prov and hopefully open on your gps.
There are other threads on here, old and more recent with the same issue you are having, heck I had it before. There are similar and unique reasons for problems, but we will figure it out. Having a gps and getting maps to it requires some patience.
Report back with a progress report
Sockmonkey