Apple's tool for reinstalling macOS on devices without using local media is a fail-safe function that allows admins to reinstall or upgrade the OS. It's the first step to recovery during deployment.
Many versions back, Apple stopped providing physical media versions of its operating system, then titled OS X. By moving to a completely digital distribution model, Apple allowed admins to download a clean copy of the OS whereby it could be used to install or upgrade a device, as needed. In doing so, admins could also create an offline installer that could be copied to a USB flash drive to facilitate installations on devices with corrupted OSs or replaced storage drives, requiring a clean installation to be performed.
In addition, each Mac with a modern version of macOS/OS X installed on it comes with the capability to upgrade or restore its operating system through a wired or wireless network connection directly from Apple's servers—without needing to first download the standalone installer or create an offline USB installer—with all necessary components downloaded over the internet first to a temporary partition before commencing with the local installation.
Through the latter option, known as Internet Recovery, Apple provides not only a current, clean copy of macOS for installation on compatible devices, but also seamlessly integrates firmware updates and, if enrolled in Apple's Device Enrollment Program (DEP), the management profile that points the newly restored device to its proper MDM server for further zero-touch management moving forward.
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There are several options available to admins and users alike wishing to install or upgrade macOS via Internet Recovery, all of which depend on which versions of macOS they're trying to install, based on the model of their Apple computer and which OSs are supported on specific devices. Below we'll go over the three methods used to invoke Internet Recovery for each scenario.
How to install or reinstall the existing version of macOS
Keyboard combination to hold during boot: Command-R.
This method is the most common and is used to reinstall macOS based on the most recent version of macOS installed on the target device. This is useful when performing a factory reinstall of the OS, such as to provision an existing device for a new user or to initialize the device for use with DEP.
How to install or upgrade to the latest version of macOS
Keyboard combination to hold during boot: Option-Command-R.
This method should be used when performing a clean installation, typically when upgrading or replacing a storage drive, or when upgrading to the latest version of macOS supported by the target device. In the event of a corrupt operating system, the second method provides the capability of installing macOS cleanly.
How to install or reinstall the version of macOS that came preinstalled with your Mac
Keyboard combination to hold during boot: Shift-Option-Command-R.
The main difference between this and the options listed above is that this process will not install the latest version of macOS nor will it provision the last known installed version of macOS. Instead, this third option will attempt to install the version of macOS (or OS X) that came shipped with the device from factory or the next available version.
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