It happens: just when you think it’s safe to update macOS, it turns round and bites you on the bum. Instead of getting the latest, fixed and sweet-running version, Software Update dumps its ghost on you, and macOS is staggering along looking like it’s half dead. So what do you do next?

The first and most important action is not to rush at trying to fix it, or give up in disgust and start reverting to the last version you were running. Updating macOS is one of the most complex things that happens to your Mac. Although mis-updates used to be quite common in the past, they do still occur now. If you were just about to go out, for example, don’t rush at anything in a panic. If macOS does really seem messed up, shut down or leave your Mac until you have counted to a hundred, had a strong cup of your favourite beverage, and have worked out what to do next.

When you’re ready, in most cases the best thing to try next is to start up in Safe mode, which is explained in full here. We often don’t do that properly, and it’s worth following Apple’s instructions very carefully:

1. If your Mac isn’t already shut down, shut it down, and wait at least 10 seconds before pressing the Power Button to start it up; restarting is not a good way to get this to work properly, although on many occasions you may get away with it.
2. As soon as you have pressed the Power button, press and hold down the Shift key on your keyboard.
3. Release the Shift key when the login window appears. Although in most cases you can release it earlier, that can risk performing a regular startup instead.

In Mojave and High Sierra, starting up in Safe mode can take an age, as macOS performs a full check on all APFS snapshots first. This has changed in Catalina, and full details of what happens there are given in this article. Safe mode does two things which are likely to help: it clears a lot of caches and other things which could be causing your problem, and it disables third-party extensions which could also be an issue.

If Safe mode seems to fix the update so that most things are working properly again, and you can’t think of any incompatible extensions etc., then try restarting back into normal mode. If your problems recur, then you may have a third-party extension or service which is incompatible, and you need now to disable all suspects, to try to isolate the one that is at fault.

If Safe mode makes little or no difference, the next thing to try is downloading a standalone version of the update and installing that. If this was a macOS update (rather than a Security Update), you should make this the latest Combo updater, which will install all the system files which have been changed since the original release of that major version of macOS. Locate that updater by searching Apple’s Support section, with the macOS version that you need and the word ‘combo’ as your search term, e.g. ‘10.14.6 combo’. Catalina 10.15.2 Combo updater is available here.

If that fixes your problems, you will also need to bring your Mac up to date with respect to any subsequent ‘silent’ security and other updates. The simplest way to do that is using either of my free utilities SilentKnight or LockRattler. Otherwise, if you don’t use Terminal, you may be able to force that by opening the Software Update pane, clicking its Advanced… button, unchecking the Install system data files and security updates item, then enabling that again.

If you’re prepared to type in a command in Terminal, enter the following command from an admin user account:
softwareupdate -ia --include-config-data
and press Return. Once those updates have been installed, they’ll be reported in the Installations item of System Information. Sometimes they can take a while to happen.

Sometimes, even a Combo update doesn’t fix serious problems. These may resolve after re-installing macOS, or even performing a clean re-install, as discussed here and in this article. In Catalina, one unique problem which can occur is that spurious additional volumes are created by the installer. To discover whether that has happened, check your boot disk using Disk Utility.

If none of these sorts your problems out, then there are still plenty of other things which you can try before admitting defeat and going back to the last version of macOS. These include starting up in Recovery mode and running First Aid from Disk Utility there on your startup volume, and resetting the SMC and NVRAM.

It’s also worth bearing in mind that your problems could result from a hardware issue, either with your Mac or with a connected peripheral. Hardware faults in Macs can sometimes wait for a macOS update before making themselves apparent, so you could perhaps run Apple Hardware Test or Diagnostics, detailed here. You can eliminate peripherals from the question by shutting down and disconnecting all non-essential peripherals, ideally leaving just a wired USB mouse/trackpad and wired keyboard, any required display, and any wired network connection.

Remember, if you have Apple’s rechargeable wireless keyboard or trackpad, you can turn those into wired devices by simply plugging their charging cables into a USB port on your Mac. If you only have Apple’s rechargeable wireless mouse, you will need a wired USB mouse to use instead, as you can’t use the Apple mouse while the recharging cable is connected.

One potential source of diagnostic information during all this are the log entries in the unified log. You cannot browse what happens during startup, though, using Apple’s bundled Console alone, as it doesn’t show you any entries from the past. There are two solutions: you can either save your log as a logarchive and browse that in Console, or you can use my free log browser Consolation.

The unified log introduced in Sierra now contains a great deal more than in previous versions of macOS, sometimes tens of thousands of entries each minute. You’ll find some helpful information about what to look for when navigating the log in this article, and this account of startup in 10.15 may be a useful reference.

You might also consider whether you may have a munged preference file or permissions. This article looks at how you can repair those, although they’re normally only responsible for little niggles, not complete failures of obvious functions in maOS. The repair process is diagrammed here.

Another trick which sometimes comes in handy is to create a new user account, preferably with admin rights, and try starting up and logging into that account. If your problems vanish, then it makes it extremely likely that they are caused by something – such as a startup item, LaunchAgent, or LaunchDaemon – in your normal Home folder.

You may well not be alone in your problems. I try to feature timely information here about known problems with updates, and discussions on Macintouch and other forums can be very helpful. Apple’s support discussions can sometimes attract expert solutions, and you can always use the Apple Support app on iOS, tweet @AppleSupport, or arrange an appointment at your nearest Apple store. If you’re having problems with macOS or any other Apple product, it’s helpful to everyone if you take them to Apple Support, as they can then be reported back to Apple’s engineers. You can also do that yourself through Apple Product Feedback or, if you have a developer account, the new Feedback Assistant, available as an app for Catalina.