Cloning internal disk with Catalina to partition on external USB

Yes, thank you. I did erase the destination while booted in Catalina. APFS was the only format Disk Util allowed. I erased the internal drive before cloning a bootable High Sierra volume back onto it. Interestingly, though the HS volume was made earlier, from a non-APFS disk, it cloned onto the internal drive without incident and runs smoothly. Using CCC Version 5.1.28 (6213)

Thank you. I'm familiar with the basic principles. My question was more specific: How can I get Carbon Copy Cloner to clone a volume with this dual drive OS to an external volume? The version of CCC I have will only allow one volume as source and the internal disk on which Catalina resides shows up in the Finder and in CCC as two volumes, a"container disk" which cannot be selected as a source in CCC Version 4.1.24 (4660).

I finally have things as I would wish. Catalina is parked on an external drive just in case I need to boot in an OS that won't run many of my principal applications. High Sierra is now my internal boot drive. Much of this is due to good advice I got here, and some due to computer voodoo. Example: I never did figure out why the command and option keys on my Bluetooth keyboard had abruptly decided to swap actions, command behaving like option and vice versa, but when, for some reason unknown, settings in the modifier keys part of the keyboard preferences finally decided to stick instead of reverting every time the panel was opened, I was able to deliberately choose opposite actions for the command and option keys and they perversely began to operate in the normal way. Solutions like that are never entirely satisfying because they are not understood. Voodoo!

Thanks. Sometimes a person just seems to need to hear the obvious.

Something like that.