[Darryl Ramm]

Uh why would you upgrade to Catalina if you did not want or need to?

If you have the disk space you can install both Mojave and Catalina on the internal drive.

If you do not *need* two different boot environments this is a lot of extra work and potential problems you can create for yourself.

And as Jack has already said, all you sound like you want to do now is clone your internal drive to the external, so Carbon Copy Cloner is the easy way to go.

Now if you are going to run off an external drive, where/how are you backing that up?