FileVault’s full disk encryption is a really nice feature of macOS, and making a bootable full-disk daily backup of your Mac’s internal hard disk or SSD using Carbon Copy Cloner or SuperDuper! is a great way to avoid downtime if your internal hard disk dies. You just reboot from the backup drive and get back to work, having lost at most 1 day of work.
However, you may encounter trouble when trying to create an encrypted bootable volume on your external backup drive. I had no trouble finding helpful articles to tell me how to reformat the external drive as encrypted, or even to convert the disk non-destructively to an encrypted volume, but none of them actually worked. In every case, the UI was happy to say it was gonna encrypt the drive and even gave me a recovery code and took a passphrase for the encryption, but then failed with the following message:
Unable to create Core Storage Logical Volume Group.
I tried the command-line method described here (which boils down to diskutil cs convert /Volumes/your_drive_name_here -passphrase) and that failed too, although it gave me a numeric error code Error -69784 in addition to the message.
I tried a bunch of different things (erasing the drive, running the installer from a bootable SDCard so as to create a recovery partition on the drive before booting from the external disk and enabling FileVault from there, and repartitioning the drive in various ways) and none of them helped. It always failed in the same way.
What did help was the “Can’t create a new CoreStorage Logical Volume on an External SSD” thread on discussions.apple.com, which had the answer: use a different enclosure. I didn’t like that answer, and tried the other stuff to avoid spending any money, but ultimately it did solve my problem.
So, if you’re stumped by Error: -69784: Unable to create a new CoreStorage Logical Volume Group, try a different external hard disk enclosure (with the same drive you were trying to use before).