Booting the Mac: Will my Mac boot from this disk? A visual guide

Lots of things can, of course, go wrong with booting a Mac. But amazingly, almost every time, your Mac breezes through the complex sequence of phases, loads the kernelcache up, and off it goes. One of the most frustrating problems is when you can’t get your Mac to start up from a given disk, typically an external drive. This article offers some guidance as to what can and does go wrong, and how to tackle it.

This week’s visual guide is based on the sequence of stages which occur when a Mac boots, which appears in the pink boxes towards the left. To the left of those are the standard user controls over booting, such as the Option key to enter the Startup Manager.

In the centre, shown in red and pale orange, are the problems which are most likely to arise, and their data dependencies. Then at the right, in green, are the steps which you can take to address those issues.

I don’t insult your intelligence by stepping through the most fundamental and common-sense issues, such as checking that an external drive is properly powered up and physically connected. Nor should you see the steps at the right as a fixed sequence of manoeuvres to be performed whenever there’s a problem. But by careful observation and thought, you should quickly work out what to try next.


I hope that you find this helpful. If you do, please let me know and I will look at summarising other troubleshooting procedures in similar diagrams.

You are very welcome to keep and pass on copies of this diagram. If you wish to reproduce it (or any derivative of it) for commercial purposes, please discuss with me first. I also greatly appreciate being credited, and this link being passed on. I don’t make a penny out of it, but it does give me a good feeling.