Can I encrypt an external hard disc that is used for backup and is also bootable? 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.
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.