big surdisk-utilityencryptionexternal-diskhard drive
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?
Aha! The answer comes when you try to deselect the "Encrypt backups" option:
So it appears they are one and the same.
This might answer your question - though it scares me to death - https://superuser.com/questions/495672/how-can-i-convert-a-mbr-partition-to-gpt-without-loss-of-data
Alternatively, iPartition can do it - but can't handle Core Storage Volumes, so it's usefulness may be short-lived as more of these come into use. [Unless they release an update, of course]
Either way, I'd make sure to have a backup before starting.
Best Answer
Apple says you need to remove the disk from being a Time Machine destination for your Mac first.
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.