macOS installation couldn’t be completed: the fixes in order

Getting to macOS Recovery, and which macOS it installs

On a Mac with Apple silicon. Apple says to press and hold the power button as the Mac starts up and to release it at the startup options screen, which shows the startup disks and a gear icon labelled Options. From there the Mac can start from another disk, in safe mode, or in macOS Recovery. Recovery installs the current version of the most recently installed macOS; Apple adds that after an upgrade followed by an erase in Disk Utility, it might install the macOS used before the upgrade.

On an Intel-based Mac. Command-R starts the built-in Recovery and gets the current version of the most recently installed macOS. Option-Command-R starts Recovery over the internet and might give the latest macOS compatible with the Mac; Shift-Option-Command-R might give the macOS that came with the Mac, or the closest version still available. Shift alone starts up in safe mode.

When a key combination does nothing. For an Intel-based Mac, Apple says to press all the keys together, to shut the Mac down first and press them after the power button, and to wait a few seconds if the keyboard needs time to be recognised. A wireless keyboard should be plugged in if possible, with a keyboard made for a PC, Apple says to try one made for a Mac, and some combinations do not work while a firmware password is on.

The network Recovery needs. Apple says installing from Recovery needs a broadband connection that uses DHCP over Wi-Fi or Ethernet, and that some connections can prevent it, including those that use a required captive-network login, 802.1X security, or PPPoE (unless a router is handling the PPPoE connection). A hotel or office Wi-Fi with a sign-in page is the captive kind; that example is ours.

What Apple says happens when reinstalling from Recovery

Data. Apple says reinstalling macOS does not remove apps or personal data, and that a backup first is still a good idea.

The wait. The Mac should not sleep and its lid should stay open until the installation completes. Apple says it might restart and show a progress bar several times, and the screen might be empty for minutes at a time.

Two questions the installer may ask. If it asks to unlock the disk, the answer is the password used to log in to the Mac; a forgotten one is in forgotten Mac password. If it offers Macintosh HD or Macintosh HD - Data, Apple says to choose Macintosh HD.

If the installer does not see the disk. Apple says to quit it, open Disk Utility from the Recovery options and erase the startup disk; if Disk Utility cannot see the disk or erase it, the Mac might need service. A check of the drive from macOS itself is in how to check hard drive health.

First Aid in Disk Utility, the way Apple orders it

Before. Apple says to have a current backup, in case damaged files need recovering or Disk Utility finds errors it cannot repair. For the startup disk, Disk Utility is opened from macOS Recovery.

The order. Choose View › Show All Devices, if available. Select the last volume on the device, click First Aid and then Run (or Repair Disk). When the check ends, select the item above it and run First Aid again, moving up through each volume, each volume group, each container and, at the end, the device itself. A dimmed button means that item is skipped.

If it cannot repair. Apple says to erase (format) the disk. If the device does not appear in the sidebar, Apple says the Mac or the device might need service, and to shut down, unplug nonessential devices and try again.

Other ways Apple gives to get the installer

Software Update. Apple calls it the fastest and easiest way to get macOS updates, and says it shows only updates compatible with the Mac and can use less storage space than other methods.

Terminal. On macOS Catalina 10.15 or later, the command softwareupdate --fetch-full-installer --full-installer-version followed by a version number downloads the installer to the Applications folder. If Terminal says update not found, Apple says that version is not available for the Mac, and softwareupdate --list-full-installers lists the ones that are.

The App Store and a bootable installer. Apple links some older versions in the App Store, from macOS Sequoia 15 back to High Sierra 10.13, and some still older ones as disk images. Apple says a bootable installer can be useful when other methods cannot install macOS, or to install it on several Macs without downloading the installer each time.

One owner’s account, from 2019

Which Mac. Jay Versluis wrote on The WP Guru in June 2019 about a MacBook Pro from 2011 that showed the message after a High Sierra security update. What worked for him, to a certain extent, was holding the Option (Alt) key at startup and choosing the regular drive rather than the boot partition: the Mac started without applying the update. On an Intel-based Mac, Apple lists Option as the key for Startup Manager, which chooses among startup disks or volumes.

What did not. Safe mode took several hours on his 500GB SSD, he says, and did not fix the problem. Among the tips he lists, rather than results, are First Aid in Disk Utility, reached with Command-R, and checking for login items that may no longer be compatible after a software update. This is one Mac, one macOS version and one person, and the Apple pages read for this guide do not mention login items.