Downgrade OS to Mojave
I've followed MacWorld's instructions on downgrading to Mojave from Catalina (because Catalina doesn't support my high-res scanner) by booting from a USB drive that contained "Install Mojave OS" and I expected that it would erase my HD and install Mojave. Instead it installed Mojave on a new "HD-Data" and Catalina and all my apps and data (pictures, movies, etc.) are still there. But it now boots from Mojave. What did I do incorrectly? Thanks!
MacBook Pro 13", macOS 10.14