It’s unusual that I see something entirely new on my Mac. A few days ago, I got up early to get some writing done, but my 27-inch iMac’s Internet connection was being horribly slow, and the entire machine was struggling. Restarting my AirPort Extreme Base Station didn’t seem to help, so I restarted the Mac. Instead of booting normally, I ended up at a screen containing only an Installation Log window and an error dialog saying “The macOS installation couldn’t be completed.” That was confusing since I hadn’t asked the App Store app to install macOS 10.13.4.
However, clicking the Restart button just brought up the screen and error dialog again. I looked through the log and saved a copy, but none of the errors looked all that problematic.
With seemingly no other recourse, I clicked Restart one last time, held down Command-Option-R to boot into macOS Recovery, and reinstalled the operating system. (Did you know that different versions of Command-R cause different versions of macOS to be installed by macOS Recovery? Check out this Apple support article for the details.) Afterward, everything was fine, so I chalked it up to gremlins and started writing this article, but got sidetracked by work on our Internet infrastructure.
A few days later, however, my MacBook Air was performing badly, so I decided to restart it as well. I know for certain that it was running 10.13.3, and I explicitly did not ask for 10.13.4 to be installed — I just wanted a quick restart to clear up the performance problems.
Shockingly, when the MacBook Air rebooted, it showed the same Installation Log screen and error. The log looked similar, and restarting had no effect. Rather than reinstall macOS right away, though, I tried something new: I held down the Option key at startup and then selected my boot drive. That worked — the MacBook Air booted normally into macOS 10.13.3, and when I intentionally installed 10.13.4, the installation proceeded properly.
Surprising as it was to experience the same entirely new problem on both of my Macs in quick succession, I figured that I do things that most users don’t, like enable the root user to test security bugs (see “High Sierra Bug Provides Full Root Access,” 28 November 2017). But just a few hours later, my son Tristan called. This never happens — like many people of his generation, Tristan is categorically allergic to the telephone — but I hadn’t responded to a picture of his MacBook Pro that he’d sent me in Slack. When I looked, I couldn’t believe my eyes. It was the same Installation Log screen and error as I’d seen on both my Macs.
Luckily, the trick I’d employed to get my MacBook Air to boot worked well for him too — it’s always nice when we aged parents get to show off our technical prowess. Tristan also provided another data point that he had not tried to install 10.13.4 at all. In fact, his MacBook Pro had restarted only because he had plugged it back in after it had run out of power and shut down. He said that it had been running very slowly before that too.
So let’s recap. In three separate instances, a Mac that’s running macOS 10.13.3 starts running slowly. Upon restart — without the user asking to install 10.13.4 — the Mac boots into the Installation Log app and shows an error saying the macOS installation couldn’t be completed. Restarting doesn’t help, but the first thing to try is holding down Option as the Mac boots and selecting the primary drive. If that doesn’t work, boot into macOS Recovery and reinstall the operating system. No data will be lost either way, but hey, make sure you have backups anyway!
How common is this problem? Unclear, although there’s a discussion of it on AskDifferent that covers the same ground I did. If you’ve already upgraded to macOS 10.13.4, you’re probably safe. But if you’ve been holding off upgrading, which is usually the cautious thing to do, be aware that it’s possible you might encounter this situation on your next restart.
I have little idea of what could be going on under the hood. Perhaps it’s related to the “Download newly available updates in the background” checkbox in System Preferences > App Store, although that was set differently on my two Macs. It might not even be connected to macOS 10.13.4 — perhaps the “macOS installation” that’s failing is related to the “Install system data files and security updates” checkbox, which should always remain selected (see “Make Sure You’re Getting OS X Security Data,” 30 March 2016).
If you have any insight into this problem, let us know in the comments.