Encrypting a mounted APFS drive

Carl Hoefs <[email protected]> Wed, 5 Aug 2020 09:58:19 -0700
Newsgroups gmane.comp.macosx.general
Message-ID <[email protected]>
Pre-Mojave: It used to be if you had an external HDD or SSD (not the boot drive) that Finder would allow you to do an "encrypt-in-place" on it in the background. That option no longer shows up. 

Mojave and later: Finder still allows me to *decrypt* a mounted & encrypted APFS or Mac OS Extended drive, but not go the other way...

Is there a way to encrypt my APFS-formatted external drive without losing the contents?

-Carl