How do I encrypt an external HFS+ drive without erasing it on Big Sur?

I have a Time Machine backup drive, which has to be HFS+ for Time Machine to work on it, and which I don't want to erase since it has a long history of backups. I would like to encrypt it.

What I find online is to control-click it and select encrypt. However there is no such option. I can't find anything in Disk Utility or the diskutil command to encrypt a non-APFS drive.

How do I encrypt this disk without erasing it?

Solution 1:

Apple says you need to remove the disk from being a Time Machine destination for your Mac first.

If you want to change from unencrypted to encrypted backups, you must remove your backup disk and then set it up again.

https://support.apple.com/en-gb/guide/mac-help/mh21241/mac

Removing the disk doesn’t delete the contents of the disk though, so you should be able reassociate the disk with your Mac once it’s encrypted, though I haven’t tested this.