Used BaseCamp to dowload Northeast Region OSM Topo map to my new Garmin Etrex 30X . MapInstall tells me that it's installed on the Etrex but it doesn't show up when I "Select Map" on the Etrex. My archived tracks display on the Northeast Region map on the pc's BaseCamp but not the other way around.
I'm not quite clear as to what you are describing, but most likely this is the same problem everyone has now. Last May, Garmin posted an update to Basecamp/MapInstall and this version will not install third party maps. Then they pulled the plug on further development of that software, so - whether it was intentional or just a bug - you cannot use the current version to send maps to your GPS. The maps look fine on your computer, but they won't work on a GPS.
See this thread for more discussion and some solutions: https://forums.gpsfiledepot.com/index.php?topic=4539.0
I have to check this out.