You can start the internet recovery reinstall process from either a shutdown Mac, or by rebooting the Mac. Whenever possible, you should back up the Mac with Time Machine before attempting this. Internet Recovery requires internet access, that may be slightly obvious but it’s worth mentioning because if the Mac is unable to connect to a network then it will not be able to download the operating system. ReInstall OS X on a Mac Using Internet Recovery This guide is simply going to cover how to start a reinstallation of OS X with Recovery mode, and not the other options available when booted into recovery. You can always start a backup with Time Machine before if you need to. Do not attempt to use recovery mode for reinstalling unless you have made a thorough backup of your important files beforehand, doing so without a backup may result in irreversible file loss. Note: Re-installing OS X is typically only necessary when something is seriously wrong with the Mac system software and the computer is not functioning as it should. That difference in how the two features works means you could theoretically use Internet Recovery to downgrade OS X to the version that shipped with the Mac, though there are better ways to do that if you have a Time Machine backup from the prior OS X release handy. Specifically, the Recovery HD partition will re-install the most current version of OS X that is currently installed on the Mac, whereas Internet Recover will re-install the version of OS X that originally came with the Mac. Both of the recovery modes let you re-install OS X, but the version of OS X that is installed onto the Mac will be different depending on the type of recovery mode used.

To be clear, we’re going to focus on Internet Recovery here, but there are actually two modes of system recovery one of which is based on a local Recovery disk partition, and the other which is called Internet Recovery, the latter being triggered if the Mac does not have a Recovery partition found or if booted directly into as shown in this walkthrough.

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