Quote:
Originally Posted by dnanian
No real trick here. You have to enable booting from external devices in the security preferences in Recovery (Cmd+R during power on; look in the Apple menu once you're in Recovery.)
But apart from that, it'll all work as you're used to. Your backup is still a valid OS and secure boot will be fine.
Note that I do suggest formatting your backup drive as APFS.
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That was easier than expected but I think changing to AFPS is actually necessary. I was having trouble getting it to recognise a startup disk without that.