How to ensure the external disk has you as its Owner, with a LocalPolicy that allows your Mac to use it as a boot disk. When yo avoid using a DFU port, and negotiating keychain problems.
LocalPolicy
One Thunderbolt port is the designated DFU port, to be used when connecting via USB-C in DFU mode, and to be avoided for external bootable disks. But how can you identify it?
Apple silicon Macs were designed to boot almost as securely from external disks as from their internal SSD. That makes macOS installation a little more complicated. Here’s how to do that.
Working with external bootable disks: how to create and add them, ownership and LocalPolicy, how that can be changed, and what happens with errors and failure.
It took over 6 months before creating bootable external disks was fairly reliable, and even then there were unexplained failures. Did someone fail to tell us something?
Two important catches that can cause a macOS installation to fail in Apple silicon: using the DFU port, and not setting up ownership correctly. Both are explained here.
Ownership can mean two very different things on Macs: it might be about Unix permissions, or Apple silicon boot policy. How to tell them apart so you use the right solution for ownership problems.
Sometimes Apple silicon Macs refuse to boot from a bootable system. How to use bputil to check LocalPolicy and work out what’s gone wrong.
How Apple silicon Macs create LocalPolicy to allow another boot volume group to be used, the problems that can occur, and how to investigate them.
To understand what might go wrong when trying to boot an Apple silicon Mac from an external disk, start with LocalPolicy: what it is, and how it works.
