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I've already installed macOS Sierra. But I've also performed a "general" cleaning and I suspect some files were gone. Now the whole system is slow, some things doesn't behave the way they used to. Is it possible to download again macOS Sierra from AppStore and simple reinstall it? I mean, install macOS Sierra over macOS Sierra that is already up and running (not so well...). Thanks a lot for your help.

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The short answer is YES.

It is possible to install Sierra over Sierra. The procedure is no different than installing one OS over another. Personally, I would wipe/format my target drive (using Disk Utility) first. Be sure you have a backup of anything you don't want deleted forever.

There are a couple ways you can do this:

  1. Internet Recovery (Hold Command R while booting)
  2. Download Sierra from App Store and create a USB installer (preferred)

Both methods will utitimately work identically once you have the installer loaded. With option 2, if you choose to restart or want to do it all over again, you won't have to download the image repeatedly.

To create the installer, you just need an 8GB or larger USB flash drive and the installation image. Once downloaded, format the USB drive with a GUID partition, Journaled and call it Sierra (or something easy for you to remember). Then, issue the following command:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Sierra/ --applicationpath /Applications/Install\ macOS\ Sierra.app/ --nointeraction

Re-install to your hearts content.

  • I did a clean install a week ago. Is the flash drive necessary? I managed to first clean the disk and the reinstall the OS just fine without it. – Fine Man Sep 24 '16 at 18:28
  • It's not necessary per se, it just makes it easier. – Allan Sep 24 '16 at 22:29
  • Oh, OK. I thought it was pretty easy without it. :) – Fine Man Sep 24 '16 at 22:36
  • @Sir Jony, Sounds like you did an upgrade install, being the install was started from the running older OS which is not what is considered a clean install. – user3439894 Sep 25 '16 at 0:32
  • Oh, really? :) Oops. What's the difference (of outcome)? – Fine Man Sep 25 '16 at 18:02

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