Many of us have just installed the largest macOS update ever released by Apple (as far as I can tell), and there’s a few Macs which haven’t fared well. Although the matching security updates for Mojave and Catalina were far smaller, a few have encountered problems with them too. What should you do next, once the feelings of panic are under control?

Safe as houses

If your Mac can start up to a degree or better, there’s one quick trick you can try which sometimes magically fixes everything in the twinkling of an eye: start up in Safe mode. On an Intel Mac, this is triggered by holding the Shift key during startup; on an M1 Mac enter Recovery, select your startup disk, press and hold the Shift key and click Continue in Safe Mode.

Once running, leave your Mac for a couple of minutes, then restart in normal mode, with your fingers tightly crossed. If that has done the trick, you can buy me a beer when we next meet.

Sometimes Safe mode works fine, but as soon as you return to normal mode, everything goes wrong again. That’s a good indicator that something you have installed is at fault, rather than the macOS update, although of course it’s normally a combination of both working against one another. You now need to hunt down the third-party extension or other software which has become upset by the update, and either update or remove it.

If Safe mode either doesn’t help, or you can’t even enter it, then your Mac’s problems could be from the macOS update itself or something third-party, and teasing them apart isn’t going to be easy. This is when you should, for the first time, ask yourself whether you want to return to your previous version of macOS without the update, or try to fix what you’ve got. You can change your mind later, but this is a key question which determines what you do next.

Press ahead

Until Big Sur, the next step for those wanting to stay with the update was to install it differently. If you’d installed the update using Software Update, then you’d download the standalone delta or Combo update and use the Installer app to apply that update. The ultimate here was installing the Combo update, because it contained all the changed components since the first release of that version of macOS, but was smaller and simpler than a full installer.

Apple has stopped providing standalone installer packages for Big Sur, or at least it hasn’t yet decided whether it will provide them, which amounts to the same: there are no standalone updaters for macOS 11. So if your current attempt to update hasn’t worked properly, your options are limited.

Simplest and most tedious is start up in Recovery mode and re-install the current version of macOS, which should by now be the version you’re trying to run. Ensure that, when you do this, you install to your current Volume Group so that the existing Data volume is connected up with the fresh System volume. Because that may not always work, before starting this journey, ensure you’ve got a full copy of your Data volume. Carbon Copy Cloner is an excellent tool for doing that, if you don’t already have a full Time Machine backup.

There are variations too, although these bring greater risk that your current Data volume will get trashed or ignored. You can download the latest installer app from the App Store, or in Terminal, which gives you a more precise choice which is essential when you want to downgrade. The following command lists available macOS installers:
softwareupdate --list-full-installers

Currently, that list incudes:
Big Sur 11.3, size: 12425084347K
Big Sur 11.2.3 size: 12211077798K
Big Sur 11.2.2 size: 12200254955K
Big Sur 11.2.1 size: 12199403070K
Catalina 10.15.7 size: 8248985973K
Catalina 10.15.7 size: 8248854894K
Catalina 10.15.6 size: 8248781171K
Mojave 10.14.6 size: 6038419486K
High Sierra 10.13.6 size: 5221689433K
Apple unfortunately removed earlier installers for Big Sur, but now maintains a full list as above. Wise users collect a set of full installers in a safe place, to ensure that they can always go back if they need to.

You can then use a command like
sudo softwareupdate --fetch-full-installer --full-installer-version 11.2.3
to download the installer of your choice.

If you install macOS afresh with your existing Data volume and the problems persist, then it’s most likely that they result not from any error in the system update, but in third-party software. You can then rip out all third-party extensions and anything from your apps which persists, until you find the offender.

Retreat

Returning to a previous version of macOS isn’t an easy option after you’ve performed a macOS update. At one time, a snapshot of the System volume was made in preparation for macOS updates, but that seems to have lapsed, and in any case those were Time Machine snapshots, so were routinely deleted after 24 hours. So there’s no secret Roll Back button, and you’ll have to perform a fresh install of that older version of macOS, ensuring that hitches up with your existing Data volume. If the latter part doesn’t work, be prepared to migrate from a backup into a fresh Data volume.

Intel Macs have one significant limitation here: firmware. While you can roll the system back, you can’t return an Intel Mac’s firmware to a previous version. So if the problems you’re encountering are firmware-related, your Intel Mac is out of luck.

M1 Macs can readily be reverted to older firmware, though, by putting them into DFU mode and restoring an older version of Big Sur complete with its firmware, using Configurator 2. You’ll need another Mac to run that free app (from the App Store), and the first time you do this is daunting, but it’s a valuable feature which can recover from apparent disaster.

Boot cycling and serious problems

Very rarely, a macOS update may go so wrong that all the ideas above become irrelevant, as you can’t get your Mac to boot properly at all. Intel models without T2 chips can sometimes be recovered from that by booting them from an external disk, but if a T2 model’s security policy doesn’t allow that, they remain a problem which may need a trip with Configurator 2. An M1 Mac is, once again, far simpler as it means putting them into DFU mode and restoring them completely.

If you suffer any significant problems after a macOS update, don’t be afraid to contact Apple Support. Sometimes updates have serious failings with specific models, and only Apple Support is likely to discover this, and be able to offer a way forward. Otherwise, I wish you success diagnosing and fixing your problems.