It seems as though macOS Catalina won't boot through FireWire connections, so I had to use USB. This worked, and I got a clean install of macOS Catalina to boot from an OWC 6G SSD external USB drive.If you bought and use Carbon Copy Cloner, you might want to look here:
However, when I try to look at the main Catalina SSD on my 2012 MacBook Pro, it only shows me the 3rd-party applications and not the Apple applications. So, Time Machine, Preview, Mail apps, etc are not visible from my external SSD. It appears as though Apple is demanding that you not remove unwanted applications that they install on your Catalina update. Really strange, in my opinion..