One command, bless, used to do it all, copying boot.efi to the right place and declaring the volume bootable in its header. Where are we with Big Sur and M1 Macs?
You want to boot your Mac from an external drive, but it doesn’t work. A diagram to explain what can go wrong, and how to fix it.
The command bless used to be able to control all things to do with booting, but has become steadily less useful with the advent of SIP and others.