Last Week on My Mac: Some boot, but others don’t

Until the end of 2017, every Intel Mac could boot from an external disk, and doing so was a popular choice with users. It allowed us to dual-boot without touching internal storage, provided an easy means of self-rescue, and saved us from having to replace a dead or defective internal disk.

Then the iMac Pro, with its T2 chip, changed all that, and over the next year those buying new MacBook Pros too discovered the sting in their tail: by default they can’t boot from any external disk, and even when you enable that, that Mac is still dependent on the integrity of its internal storage.

Less than three years after the arrival of the first Macs with T2 chips, Apple changed it all again with its new M1 Macs. Although they are supposed to support booting from external disks as a right, it wasn’t until Big Sur 11.4 in May 2021, over six months later, that this became fairly reliable. Since then, I have installed macOS on a great many external disks and booted successfully from them. Those have included both hard disks and a wide range of SSDs, and over the last week I’ve been testing it in Ventura with seven branded SSDs from major manufacturers. Not one has failed to boot successfully from my Mac Studio M1 Max or MacBook Pro 16-inch M1 Pro.

Yet whenever I describe what has become reliable routine here, whether in Monterey or Ventura, I get floods of comments from users who can’t get it to work at all. They universally describe their Mac’s failure to create the LocalPolicy required for that external disk to be bootable, resulting in macOS’s failure to complete its installation. That’s the same problem that plagued me before macOS 11.4, which I haven’t encountered now for 18 months.

A few have been given duff information, being told that they had to downgrade Boot Security just as with a T2 model. I don’t know who’s providing that misleading story, as Startup Security Utility doesn’t provide that option for Apple silicon Macs, and in any case, since Monterey boot security can only be changed in the paired Recovery volume. If your Mac can’t boot from that disk in the first place, you can’t change its Secure Boot policy for that disk.

Over those 18 months, I’ve been unable to explain why others seem unable to do what is here so reliable as to be routine. Between us we’ve ruled out SSDs, cables, ports and procedures. I’m now left shrugging my shoulders, and admitting that I just don’t know why some are invariably successful, and others the exact opposite, no doubt cursing my good luck as their repeated attempts all fail.

The pattern of failure points to the problem lying in individual Apple silicon Macs, but without combing through their configuration in fine detail, there appears nothing out of the ordinary with Macs that can’t install an external bootable disk. And without identifying the cause of this problem, finding a solution is wild guesswork.

I’m reminded of a lesson I learned back in 1987, when I was developing for Commodore, and learned that each of its Amiga 2000 computers had a unique ‘personality’. For my work, engineers hand-picked a computer from their stock so that it would work best for my development. It appeared that computers had then reached a level of complexity that they were no longer entirely deterministic in their behaviour. Could we have reached that in Apple silicon Macs?

It’s perhaps time to start using the command tool that Apple warns “is not to be used in production environments”, bputil, to look inside the detailed contents of a Mac’s LocalPolicy, to see what may have gone astray there. In the coming weeks I will report back on my journey to discover why, when it comes to booting from external disks, not all Apple silicon Macs are equal. Before offering to join me, first read the cautionary paragraphs in the introduction to man bputil.