It should be straightforward to install an older but still supported version of macOS to run from an external disk connected to an Apple silicon Mac. After all, Apple signs all its macOS installers, and that installation must have an owner and LocalPolicy validated by the heavily protected master copy of macOS on the Mac’s non-removable internal SSD.

It was cbfiddle commenting here who suggested something might have gone awry, and launched me on what became an epic journey through Mac history leading to ultimate disappointment and frustration.

Although I’m only too well aware of the problems of installing older versions of macOS, for those innocent to them it might seem absurd that a macOS installer created and signed by Apple just a few days ago can’t be trusted to be run in a more recent version of macOS. Yet the macOS 15.8.2 Installer app isn’t allowed to run in Golden Gate 27.2, despite them being contemporaries. It’s easy to dismiss this as deliberate obstruction on Apple’s part, but I have yet to see or hear any better explanation.

I thus embarked on the age-old ritual of creating an external bootable installer, a process that Apple still documents going as far back as macOS El Capitan 10.11 eleven years ago, although curiously it has omitted Sierra from that list.

That involves formatting a USB flash drive in HFS+J and a command for Terminal that is copied and pasted from a web page.

I can’t remember when I last used a USB flash drive, but it must have been well over five years ago, since when I seem to have given away all mine to celebrate moving on to better solutions. Instead, I pulled up a spare external SSD and reformatted it in HFS+ with journalling. Really, though, still being required to format anything in HFS+ today is farcical, isn’t it? APFS went live over nine years ago, and Apple is still dependent on HFS+ for external bootable installers. I suppose it is at least an advance on 800 KB floppy disks, and might just be a rite of passage.

As for having to copy and paste commands from a web page into Terminal, I was under the impression that ClickFix attacks had made that a little risky, and surely not the sort of practice that Apple should be requiring users to do.

What I then had wasn’t actually bootable in any normal sense of the term. Instead, I had to start the Mac up in Recovery, then get recoveryOS to switch to the Recovery system on the installer volume, or so it would appear. When that failed and I was told the installer was damaged, it was hard to guess what to do next. Was that yet another misleading error message, or should I go through the whole rigmarole all over again, only to stumble at the same message?

In my quest for truth, I did repeat the process with macOS 26.7.2, another installer that 27.2 seemingly can’t trust, and exactly the same happened. macOS 27.2 did work as promised, but no one would want to do all this for an installer that could readily be run from the Finder. Thus its sole value was in demonstrating the only external bootable installers that don’t work are those for versions of macOS that can’t be installed any other way. A lengthy procedure that only works when you wouldn’t want to use it is but a waste of time.

Had the Sequoia and Tahoe external bootable installers worked, that would only have got me part of the way to my goal. The next set of hurdles would have come in setting up ownership and LocalPolicy for the new system. Although not as laborious as this first phase, it’s still not simple, and prone to failure, particularly if you’re unaware of the significance of the DFU Port in the process.

If I had to dream up an arcane sequence as a deterrent to users, I don’t think I could have bettered that required to install an older supported version of macOS on an external disk connected to an Apple silicon Mac. All because one macOS can’t trust the installer for another version of macOS.

Apple: having great AI and touchscreens is all very fine, but such basic procedures must be more friendly to users, not engineered to repel them.