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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?

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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.

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  • Also Big Sur seems to use APFS disks for Time Machine so, don't be alarmed if Big Sur converts it. It's still pretty new and we are working through all the changes as the come up, so always proceed with caution, especially with a new OS release. but @grg is a smart guy. Personally I would take his advice. Dec 23, 2020 at 1:06
  • I removed it as a Time Machine destination, however I still was left with no option to encrypt. I was able to re-associate it to Time Machine, and the old backups were retained.
    – Mark Adler
    Dec 23, 2020 at 5:01
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    The main issue here is that Big Sur doesn‘t seem to habe a way to encrypt HFS disks (or does convert them to APFS during encryption).
    – nohillside
    Dec 23, 2020 at 8:10

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