It’s one of those commonplace tasks: installing a bootable version of macOS on an external disk. If your Mac happens to be one of the new M1 models, then you may find this tricky, or even impossible. I wish this article could explain how to do it successfully, but like many other M1 owners, all that I’ve tried so far has failed. (Note that this isn’t about creating a bootable installer for Big Sur, which is widely documented now.)

There are plenty of good reasons for wanting an external bootable disk, of course. It’s been one of the favourite recovery tools for many Mac users going back for more than a couple of decades. If your Mac won’t start up properly from its internal disk, you may well be able to get it up and running from your external disk, which you can keep armed with diagnostic and repair tools for such emergencies. This has become more complicated with the advent of the T2 chip and its secure boot, but one of the advantages of the M1 Macs, so we thought, was their ability to boot from a sealed system on an external disk without having to alter security settings.

The most obvious method of doing this is to run the Big Sur installer app, and select a freshly formatted external drive as its destination. Some who have attempted this using an M1 have reported that the installation never completes; thankfully mine was completely successful until I tried to restart from it. Although I could see and select it in the Startup Disk pane, Big Sur 11.0.1 couldn’t be booted from there.

Checking the external disk gave no clues as to what was wrong. The System and Data volumes were present and appeared to be correct, although the diskutil apfs list command didn’t pass comment on whether the System volume was sealed, which seems puzzling.

Next, after updating to Big Sur 11.1, I tried cheating. Knowing that Big Sur’s software restore feature had been fixed for the 11.0.1 release, I thought I might be able to use that via Disk Utility to restore my current boot volume to the external disk. If you ever decide to use Disk Utility’s Restore feature, you must be careful what you select as the item from which you want to restore. Selecting the disk will inevitably fail because you can only restore to a disk with an identical name, it would appear. Instead, try restoring from the container instead, which should then recreate the Volume Group together.

Only it doesn’t, because Disk Utility won’t restore from a System volume whose seal is broken.

You may remember recent discussion about this issue: it seems that many Macs running Big Sur report that their Sealed System Volume has its seal broken, according to diskutil apfs list. Of course, that doesn’t really matter, as Big Sur boots not from the System volume itself, but from a snapshot of that volume, and so long as the seal on that snapshot is intact, everything works fine. Except for Disk Utility’s Restore, it would appear.

I’m getting more puzzled about the many broken seals on Big Sur System volumes. I’ve been reassured that it’s probably because something like Software Update has mounted the System volume for writing, and it will be resealed shortly. That I don’t understand, as I can’t reseal it, and I think the only thing that can is an Apple system installer. As that has only recently updated my Macs to 11.1, and apparently failed to seal their System volumes properly on completion, I’m beginning to wonder if this is just a bug.

Whether it’s a bug or not, the broken seal on my supposedly-Sealed System Volume seems to be causing problems that I’d never foreseen. And as I can’t reseal it, except by trying to reinstall Big Sur, there’s no easy solution.

So I moved on to last-ditch measures by starting my M1 Mac mini up in Recovery Mode, and using the Install macOS Big Sur feature there to set up my bootable external SSD. I couldn’t help feeling that Apple is missing a trick here in not making better use of the internal SSV: as it contains a cryptographically verified copy of macOS, why couldn’t Recovery Mode use that to install on an external disk, instead of having to download the full install from Apple’s servers every time? What took me hours of downloading would then be reduced to copying, preening and sealing.

After downloading almost the complete Big Sur installer, Recovery Mode in 11.1 decided that it couldn’t change ownership on the external volume and simply abandoned my final attempt to install Big Sur on an external disk.

If you’re running an Intel Mac and can’t get any of these to work, you still have another solution, perverse as it might seem: install Catalina on your external disk, boot from that, and upgrade it. Of course that’s not an option for an M1 Mac, it’s Big Sur or nothing.

My conclusion is that Apple really doesn’t want us installing Big Sur on external disks. If you want to install Big Sur on an external disk for your M1 Mac and can’t, then please contact Apple Support, or file a Feedback report. And if you’ve discovered the secret, please share it here.