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?