In this article, we explain how to perform a clean install of macOS. Without a doubt, it is a very useful option if your Mac has become slow or behaves strangely, crashing certain applications, or slowing down tasks.
It is also something we usually resort to, thinking that the computer may have a virus (although there is much debate about whether there are viruses and malware on the Mac that can cause the operating system to malfunction).
We are going to start showing step by step how to do it for the latest version of macOS Catalina (at the time of writing the article), before showing how to install a clean copy of a previous version such as High Sierra, Sierra, El Capitan, Mojave , Yosemite, or an earlier version of OS X.
You can find here a complete article on how to install macOS High Sierra on your Mac.
In the following tutorial, we explain how to install macOS on your Mac by updating the operating system, but if you want to learn how to reinstall it from scratch, or download old versions of macOS X, you can also refer to this article.
What tools do i need
Before you start, you should know that there are two ways to perform a clean installation of macOS. The traditional way was to make a bootable copy of the macOS installer on a Flash drive and then format your drive before installing the bootable copy on your Mac.
There is also an even simpler method that allows you to use macOS Recovery to reinstall the Mac operating system from the web. It is something that we will also see throughout the article.
One word of warning, if you want to do a clean install of macOS Catalina and you’re already running Catalina, the process is different than previous versions of macOS. Now you will find two volumes to delete: Macintosh HD-Data and Macintosh HD. We will explain what you should do about it.
How to do a clean install of macOS using a bootable USB
Before you begin, you will need the following:
- A removable USB flash drive of at least 15 GB or more to hold the installer file for our chosen version of macOS. We are going to delete it from the system, so we recommend that you make sure that you do not have any valuable data.
- Administrator privileges for the Mac
- Make a backup so that in case something goes wrong, you can roll back to a previous version of the system. We encourage you to consult in this report, how to backup a Mac.
Regarding backups, you probably don’t want to recover a Time Machine backup if you don’t want to copy all the settings and preferences associated with your old system.
Alternatively you can sync all your documents and desktop with iCloud instead of running a full backup, but make sure everything you need is in the cloud and keep in mind that you may lose system settings and app installation .
Are you ready? We started
Step 1: Format the external drive
We are going to re-download the installer file for our chosen version of macOS and proceed to store it on a USB stick or external drive. The pendrive you use must be formatted as a macOS Extended volume with a GUID partition table.
Here we explain how to format your USB drive correctly:
- Plug in the USB drive
- Go to Applications> Utilities’
- Open the System Disk Utility
- Select the drive and click Erase. (The disk name must be ‘Unnamed’ if the following Terminal commands work, so change the name if necessary.)
- Select macOS Extended (Journaled) as the format type.
- Choose the GUID partition map from the schema options. If you don’t see these options, you would be in the wrong place. You should select the external drive, not the container under it.
- Click delete-
- Disk Utility will create the partition and configure the drive (may take time).
- Click done and you will have it.
Step 2a: Get the macOS setup file
If you are reinstalling the latest version of macOS, or installing a new version of macOS, the next step is relatively easy, although the process to obtain the installer will be different.
If you are running Mojave and want to do a clean install of macOS Catalina, you can download the installation package through ‘System Preferences’, the same will apply if you are running Catalina and want to do a clean install of Big Sur (once starts):
- Open system preferences.
- Choose ‘Software update’.
- Download the Catalina installation files or the Big Sur files.
If you want to install Catalina and you are already running the operating system, or if you are not running Mojave, the best way to access the installer for Catalina is through the Mac App Store. Here’s a direct link, but alternatively open the Mac App Store and search for Catalina.
Once you have located Catalina in the Mac App Store, follow these instructions:
- Press ‘Download or Get’ (depending on the version of the Mac App Store you are using).
- 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.
- Enter your Apple ID and password.
- The Catalina installation files will begin to download.
The installation file will be several GB, Mojave was 5.7 GB for example, so the download may take a while.
- When the macOS installer has finished downloading, the installer will start automatically, but we don’t want to use it yet, so don’t click start installation just yet.
- Press Command + Q to exit the installer
- Find the installer file (it will be located in your Applications folder, or you can find it using Spotlight (command `space and start typing Catalina or Big Sur, depending on your case).
If you just want to install Catalina, you can skip the next step, where we’ll show you how to dig up the installer file for an older version of macOS or macOS X.
Step 2b: Get the setup file for an older version of macOS
For older versions of macOS and Mac OS X, it’s a bit more difficult to get the setup file – Apple doesn’t offer downloads of outdated versions of the OS through the regular store, and looking for the name, you won’t find what you’re looking for.
Depending on the version of macOS you are running, the installers for some versions of macOS can be found in the ‘Purchased’ section of the App Store.
Try logging into the Mac App Store with the Apple ID you used and clicking ‘Purchased’ in the menu bar at the top. Scan the list of apps that you have purchased or downloaded for free and find the name of your chosen version. Click download.
There are a few reasons why this may not work for you:
- If you’ve already installed Mojave or Catalina on your Mac, you won’t be able to easily search for older versions of macOS from the Mac App Store.
- You won’t find the version of macOS your Mac shipped with in the ‘Purchased’ section of the App Store.
- You also won’t find macOS versions in the purchased section if you never installed that version.
Fortunately, it is still possible to get some versions of macOS if you are running a version newer than Sierra.
As in the previous step, once you have downloaded the installer, don’t run it, just close the installer if it opens.
Step 3: Create a bootable USB disk
Next, we must create a bootable pendrive to be able to install a new copy of macOS on your Mac from the USB. We are going to create the boot drive with Terminal, which you can open by pressing ‘Command + Spacebar’ and typing ‘Terminal’.
Connect the removable drive to your Mac and make sure it has the same name that you named it when you created it.
Create a bootable USB disk with Terminal
Open the ‘Terminal’ application on your Mac using the key combination ‘Command + Spacebar’ and type the word Terminal.
Now copy and paste the following text in Terminal (command line):
sudo / Applications / Install macOS Mojave.app/Contents/Resources/createinstallmedia –volume / Volumes / Untitled – / Applications / Install macOS Mojave.app
Enter your password (you will not see it appear). When the boot drive is ready, you will see the word Done. It may take some time. Be patient.
How to install from a DVD drive
Finally, we are going to show you how to install a copy of macOS from a drive or disk reader.
With the boot drive connected, start or restart your Mac while holding down the Option key (also known as Alt). This will take you to the ‘Administrator’ home screen. Choose to install the macOS version from the external drive.
Select ‘Disk Utility’ and the disk on which it will be installed. It will be deleted. Go back to the main menu and choose Install macOS.
Once the installation is complete, you can restore applications and other settings from the backup you created, or download them manually.
Remember not to recover a Time Machine backup if it is a different operating system version, in order not to copy all associated settings and preferences.
Step 4: Clean up your Mac and install macOS
Now we are ready to install the copy of macOS from the installation drive on our Mac. If you want to do a fresh installation (and completely erase your Mac), it’s time to reformat your Mac’s startup drive.
Here’s how to do a fresh install of macOS if you’re not already running Catalina:
- Plug in your boot drive.
- Start or restart your Mac while holding down the ‘Option’ (Alt) key. This will take you to the ‘Startup Manager’.
- Choose to install your chosen version of macOS from the external drive.
- Select ‘Disk Utility’.
- Select your Mac’s startup disk, probably the one called Macintosh HD or Home.
- Click ‘Delete’.
- Wait for your startup disk to format
- Go back to the main ‘Utilities’ menu and choose ‘Install macOS’.
- Choose your startup drive as the location to install macOS
After the installation is complete, you can restore applications and settings from your backup, or download them again manually.