I have a fresh SuperDuper backup of Mojave and booted to it. Then I tried to install Catalina. The process completed and I booted into the external HD, but it didn’t appear to work. Is this not the way to install the OS on the external hardware?
Although I haven’t tried installing Catalina release over Mojave on an external drive, I had been running the beta that way – using Superduper to make an external clone, booting to the external, and then upgrading to Catalina (beta) on that.
When the process completes (at least in my case) it leaves me at the login prompt of Catalina. If you have the login prompt for Mojave maybe it booted the internal? Anyway, from Mojave you should see the mounts for Catalina and Catalina - Data on your desktop if it actually did the install.
After it completes the install part on the external disk, I held down the Option key. I chose to boot into the external disk that started the Catalina install and it just acts like it didn’t install anything. So I tried again, this time after it needed to reboot, I let it go instead of holidng down the option key, that seemed to be the ticket. At least for now…
Yep. Don’t hold down the Option key. In fact I’ve noted that at least on my new iMac you have to select the boot disk from System Preferences. The Option Key no longer works. Probably a security measure.