I succeeded in backing up Catalina to the 1.5 TB drive, with ~400 GB to spare, and it is fully encrypted.
As others have noted, it is no fun booting from a HDD. Especially annoying is that you can't open the Security & Privacy preference pane to turn encryption on; it times out and gives an error.
I think the problem is that XProtect is busy scanning the entire drive, which takes hours on an HDD (and that's even though presumably after the initial SuperDuper clone, the HDD is perfectly defragged!).
I finally got the Security & Privacy pane to load by using the
renice command to bump up its priority:
- Open System Preferences
- Open Terminal (which in itself can be a struggle)
- ps ax | grep Preferences to get the PID of System Preferences
- sudo renice -n -10 "pid" where pid is the PID found in step 3
- Select Security & Privacy