Mac 911 is a place you can come with no judgement, even if you managed to erase your Mac’s startup disk and you’re not sure what to do next. It happens! (In the spirit of confession, I managed to delete the core operating system off my first Unix system in 1994, because I thought, “This file takes up too much space!”)
Fortunately, Apple has you covered with macOS Recovery. You may know this tool, introduced way back in Mac OS X 10.7 Lion, as a way to start up your computer to run Disk Utility without having your startup drive mounted, to reinstall macOS, and for a number of other technical purposes.
macOS Recovery occupies a small, invisible partition on your startup drive, and lives quietly there until you need it. But what happens if you erase the entire drive, including this hidden partition?
Macs can reach out over the internet and bootstrap themselves back into operation, as the Mac firmware—software built into a Mac that’s persistent and isn’t dependent on the operating system being installed—knows how to connect to the internet and reach Apple’s servers.
Start up your Mac from a shut down state or restart it, then immediately hold down Command-R. The Mac should recognize that there’s no macOS Recovery partition installed, show a spinning globe. You should then be prompted to connect to a Wi-Fi network, and you enter a password.
At this point, the Mac should start to download the recovery files from the internet and show a status bar. When complete, it should reinstall macOS Recovery and reinstall macOS on the main partition. (If your main macOS partition had system files become damaged or missing, your other files will remain intact, as this reinstallation only installs and replaces system files and apps.)
Command-R should reinstall the latest version of macOS that you had previously installed on this computer. However, you have other keystroke options depending on which version was previously installed.
Apple changed the start keys with macOS Sierra version 10.12.4. If you have ever installed that version on your Mac, no matter what version (or no version) is currently installed, you have these options.
Command-Option-R: Upgrade to the latest version of macOS that works on your Mac, which may not be the very latest release of macOS for older Macs.
Command-Shift-Option-R: Reinstall the version of macOS that your Mac shipped with.
With older versions of macOS:
Command-Option-R: Reinstalls the version of macOS that your Mac shipped with.
This Mac 911 article is in response to a question submitted by Macworld reader Luke.
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