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.
Mac
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.
Single-diagram summary of how a modern Mac starts up in macOS 10.12 or 10.13, from pressing the Power button to running the kernel.
Important info about resetting the NVRAM with a firmware password, and new procedures for resetting the SMC in Macs with T2 chips.
What boot.efi does to prepare to load and run the kernel: building the hardware device tree, handling startup key commands, and more.
The Mac has completed its self-test routines, and now prepares to load the EFI firmware or ‘boot loader’. Where it finds it, what it needs, and how Secure Boot changes this.
What happens in your Mac when you press the Power button? Before it loads the EFI firmware, it goes through self-testing, and initialising key chips.
By default, your Mac’s cooling fans run at full blast. Only the control system slows them down when it is safe to do so. How to diagnose and fix its problems.
All about firmware, the SMC, NVRAM, and the new T2 chips which come in some models of Mac. When and how to reset, and more.
It was easy to decide that this iMac needs to be replaced. Working out what to replace it with, proved more complex. How much would that iMac Pro cost?
The Eclectic Light Company 