Mac & Apple

How to clean install macOS Big Sur, Catalina or older

In this article we’re going to show you how to perform a clean install of macOS. This is often a good option if your Mac is running slow or otherwise misbehaving.

It’s also something computer users often resort to when they are trying to get rid of a virus (although there is a lot of debate about whether there are viruses on the Mac).

We’ll start by showing how to do this for the latest version of macOS (macOS Big Sur at time of writing), before moving on to the far tricker business of installing a clean copy of an older version of macOS (Catalina, High Sierra, Sierra, El Capitan, Mojave, Yosemite or an old version of OS X). Speaking of which, if an oild version is what you’re looking for, it may be worth taking a look at another article where we focus on that entirely: How to download and install old versions of OS X on a Mac.

Before you start, you need to know that there are two ways to do a clean installation of macOS. The traditional way was to make a bootable copy of the macOS installer on a Flash drive or USB stick and then reformat your drive before installing the bootable copy on to your Mac. There is also an even simpler method that allows you to use macOS Recovery to reinstall the Mac operating system over the web. We have a separate tutorial about doing a clean install of macOS using recovery.

A word of warning: If you want to do a clean install of Big Sur or Catalina and are already running Catalina the process is different to earlier versions of macOS. You will now find two volumes to delete: Macintosh HD-Data and Macintosh HD. We’ll explain what you need to do about that below.

How to clean install macOS using a bootable USB

Before you begin you’ll need the following:

  • A 15GB or larger removable USB flash drive to hold the installer file for our chosen version of macOS. (We’re going to erase it, so make sure it doesn’t contain any valuable data.)
  • Admin privileges for the Mac.
  • A back up, just in case something goes wrong. Full instructions here: How to back up a Mac.

Note that with regard to backups, you probably won’t want to recover a Time Machine backup if you don’t want to copy over all the setting and preferences associated with your old system. You could alternatively sync all your Documents and Desktop with iCloud rather than run a full backup, but make sure that everything you need is in the cloud and be aware that you may lose system setting and applications.

Ready? Let’s get started.

Step 1: Format the external drive


We’re going to re-download the installer file for our chosen version of macOS and store it on a USB stick or external drive. But before you do that you can prepare the USB drive by reformatting it.

Here’s how to format your USB drive correctly:

  1. Plug in the USB drive.
  2. Go to Applications > Utilities.
  3. Open Disk Utility.
  4. Select the drive and click Erase. (The name of the disk needs to be “Untitled”, if the Terminal commands below are to work, so rename it if necessary.)
  5. Select Mac OS Extended (Journaled) as the format type.
  6. Choose GUID Partition Map from the Scheme options (if you don’t see this you are in the wrong view – you need to select the external drive, not the container below it).
  7. Click Erase.
  8. Disk Utility will create the partition and set up the drive (it can take a while).
  9. Click Done.

Step 2a: Get the macOS install file


If you’re reinstalling the latest version of macOS, or installing a new version of macOS, the next step is relatively easy although the process for obtaining the installer will be different.

If you are currently running Catalina and want to do a clean install of macOS Big Sur then you can download the installation package via System Preferences.

  1. Open System Preferences.
  2. Choose Software Update.
  3. Download the Big Sur install files.

If you want to reinstall Big Sur and are already running Big Sur the best way to access the installer for Catalina is via the Mac App Store. Here’s a link to Big Sur on the Mac App Store. If it’s Catalina you want you can get that here. Alternatively open Mac App Store and search for Catalina.

Once you have located Big Sur on the Mac App Store follow these instructions:

  1. Hit Download or Get (depending on the version of the Mac App Store you are using).
  2. You may get a warning message at this point, telling you that this version of macOS ‘is already installed on this computer. Would you like to continue?’ Click Continue.
  3. Enter your Apple ID and password.
  4. The Catalina install files will start to download.

The installer file will be multiple GB. Big Sur is 12.6GB, for example, so expect downloading to take a while.

The next step is worth highlighting for what you shouldn’t do, rather than what you should do:

  1. When the macOS installer has finished downloading the installer will launch automatically, but we don’t want to use it yet so don’t click to st