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I have an external hard disk that serves two purposes:

  • It contains a fully functional bootable macOS installation for use should my main installation of macOS become corrupt in some way.

  • Since it's a 1TB disk and macOS takes only 20GB, I use the remaining space to back up folders by manually copying them over in Finder. I don't use Time Machine.

I'm thinking of encrypting this external disk, from Disk Utility (when booted from the main copy of macOS — on my internal drive).

If I do so, will both installations of macOS be able to decrypt it? Or should I leave it unencrypted if I want it to work in both the roles mentioned above?

I'm running Sierra, in case it matters.

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    I have a better idea: you can leave the external HD as-is with the macOS installation, and create an encrypted disk image (or sparse bundle) for your backup files. This way your backup files are secure, without needing to encrypt system files.
    – At0micMutex
    Oct 12, 2016 at 16:14
  • Great idea, upvoted, but I've had problems years ago with FileVault 1, which used these. Even now, Google searches show multiple reports of sparse bundle corruption. The data is not sensitive personal information, so better to have them unencrypted than risk losing it.
    – Kartick Vaddadi
    Oct 13, 2016 at 4:33

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You can unlock the disk while booted from other partitions. You'll be prompted for a password when mounting the disk. You can choose to save the password in your keychain if you prefer.

Also, you can just right-click on the volume in the Finder and select Encrypt "Disk Name", it's the same as using Disk Utility.

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  • To confirm, it doesn't matter whether I encrypt the external disk when booted from the external disk or the internal disk? I can always type my password and save it.
    – Kartick Vaddadi
    Oct 14, 2016 at 6:18
  • Nope, it'll be the same either way. If you're booted from the disk you can use System Preferences to enable FileVault to get a recovery key and add other users. But if you just encrypt it in Disk Utility or the Finder then I think you'll get one password (a disk password?). I haven't tried in a while...
    – MacManager
    Oct 14, 2016 at 15:39
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Another "less work" idea: Use CarboCopyCloner or SuperDuper! to clone your internal disk to the external disk from time to time: it will be an exact copy of the internal disk and bootable: the OS is up to date and configured as the internal disk, and your files ditto.

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