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I was recently having some problems with iTunes, and decided to reinstall macOS Sierra. I held Command+R on startup and clicked on "Reinstall macOS" without erasing the hard drive in Disk Utility. Once the reinstall was complete, I started back up and all of my files and programs were still there, along with my settings, desktop wallpaper, and everything else. (The iTunes problems were fixed.)

What exactly does this "Reinstall" feature do, if it doesn't remove programs and change settings to default?

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2 Answers 2

Reset to default
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It does exactly what it says it does–reinstalls macOS itself. It only touches operating system files that are there in a default configuration, so any preference files, documents and applications that are either changed or not there in the default installer are simply left alone. No doubt it is a bit more complicated than that but all a re-install does is replace the old files with "fresh" versions.

No doubt there are preference files and other things that are deleted/modified but Apple seems to have gone to great lengths to ensure that reinstalling the operating system won't delete anything that you have subsequently installed/modified.

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    As simpler way of putting it: Reinstalling is basically doing the same thing as an update would do, except you're updating to the same version.
    – At0mic
    Mar 19, 2017 at 17:33
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This installs the system that came with your Mac when it was new. It isn't associated with your Apple ID, so the new owner can use the App Store to upgrade to a later version using their Apple ID.

for more info visit https://support.apple.com/en-us/HT204904

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    No, it doesn't install the system that came with the Mac when it was new. This is a 2010 iMac with Sierra installed, but it came with Snow Leopard. I reinstalled Sierra.
    – brendon-ai
    Mar 19, 2017 at 13:32
  • on reinstall MacOS it booted with sierra?
    – baswaraj
    Mar 19, 2017 at 18:09
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    Yes, it did. This reinstall utility seems to be part of the OS itself, rather than some low-level BIOS tool. I'm curious what implications this has on reinstalling when your OS is corrupted.
    – brendon-ai
    Mar 21, 2017 at 0:34

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