I don't see it in mentioned in the article but could be a driver issue for the enclosures of theses disks. Had that happen when I was using shitty cables for an SSD to my Raspberry.
Otherwise it's entirely possible it's for security reasons and somehow related to the secure enclave stuff.
But I might way out of my depth here.
What is the use case for using an external boot disk with a mac?
I once worked with a team that worked three 8 hour shifts and flexible seating, so there were about 3 times as many workers as there were desks and each team member might be sitting at several possible computers. Rather than worry about syncing personal files and profiles across a network, each team member had their own external drive that they used to boot the Mac mini wherever they were sitting. There may have been better solutions for that particular team, but this certainly worked and I’d hate to throw away the option for doing something like that in the future.
And I’ve booted Linux from an external drive a zillion times on a PC. It’s fairly handy to have the option.
Performance kinda sucks unless you pay a lot of attention to buying the right model of drive. Most drives are simply designed for storing a few TB's of photos and videos. As soon as you are doing low latency paging to the drive and millions of flush() operations, things bog down pretty badly, even on a modern SSD.
Simple things like USB not having any decent way to do shared memory with the host are a big part of it...
I don't even think it should have to be justified. Booting from external media is one of the most basic computer maintenance tools we have. It's not like the computing industry has been giving two shits about user freedom anyway. The cynic in me wants to say that the insecurity of modern computing was intentional, so as to eventually require such user-hostile lockdowns. However, the rationalist in me understands that it most likely just ended up happening this way by chance. Either way, it's unfortunate that everything is being locked down more and more in the name of security. At the very least, such walled gardens should be opt out.
One thing that comes to mind is that as a developer you don't have to pollute your main partition with a beta if you can install it on an external drive.
AFS has containers to install betas on the same partition, the main partition is not polluted (and the additional volume(s) can be removed at will). cfr. https://support.apple.com/en-us/HT208891
I'm not saying that it's "normal" not to be able to boot from external devices, I'm just answering your point
This is quite risky - previous beta updates (such as Big Sur) did break installing updates on older OS releases in the same APFS container, and some beta updates have in the past done incompatible APFS updates that broke the ability to boot old OSes.
The fact the errors occur before the "reboot" part of the installation tells me it isn't a driver issue. The "unable to set startup disk, SDErrorDomain 104" suggests the error is a check in firmware preventing this.
It's probably simply unimplemented, and rather than try anyway there is a check somewhere in the firmware that the drive being marked bootable meets some criteria. Clearly the internal drive meets that, and some external drives do. Running a tracer on the commands sent to the drive would likely identify it.
- Booting to recover your data when the system has trashed itself, typically due to a failed software update, to the point that it won't boot off the internal drive but the drive is still working. This sort of thing has been seen with older MBs.
- Continuing to be able to use your laptop when the internal drive has failed. Very helpful if you can't afford a replacement, or if you need to carry on without waiting for a replacement and you have a recent backup.
- Booting older versions of MacOS (that don't know about APFS, or if you have reason not to trust APFS partitioning) for testing, compiling for an older target, running software that doesn't run on the current version of MacOS, or other reasons.
(But old MacOS won't work with the M1 so that doesn't matter at the moment.)
Otherwise it's entirely possible it's for security reasons and somehow related to the secure enclave stuff.
But I might way out of my depth here.
What is the use case for using an external boot disk with a mac?