Read maps from the USB pendrive

Yes, I have the old US Topo from CD on my iMac. If I understand correctly, you have already gotten the Topo onto your USB drive in a form Basecamp can read. If so, just copy it to ~/Library/Application Support/Garmin/Maps and Basecamp will read it from your hard drive. This folder is hidden in newer versions of Mac OS (I'm on 10.13.6), so use the Finder Go menu to get into it using the path I typed above.

I am on 10.15.7 and there is no Maps folder inside Application Support - Garmin. :-(. Getting crazy! 

Be sure to start the path with the tilde ~. There are two Application support folders on a Mac. If you skip the tilde you will go to the wrong one. In the Finder, drag  down the Go menu to Go To Folder and type this in exactly: ~/Library/Application Support/Garmin/Maps

PS: If you are in the correct folder, just create a Maps folder and copy Topo from your pen drive. I think this should work.

Danham, nothing to do. That path took me to the same folder as before, I tried to create a new “Maps” folder but Basecamp does not see the maps in it. My maps are .img, while yours are .gma, probably that is the difference. I do not really know what to do, I also thought to buy the Topo on the Garmin website but I think I will face the same issue. 

You should be able to copy the map file for the topo map to your pen drive just like you did with the freizecarte map.

I have done it but, once the fenix is connected, the maps reading goes very slow. It seems like the connected device overtakes the usb pen drive.