In this article we’re going to show you how to perform a clean install of macOS. There are a number of reasons why you might want to do a clean install:
You might want to reinstall the operating system in order to correct problems with your Mac – doing a clean install may be a good option if your Mac is running
slow or otherwise misbehaving.
Doing a clean install allows you to clear away old files that you might have been accumulating for years before installing the operating system onto a freshly formatted disk.
A clean install is also something you should do before selling your Mac or passing it on. It allows you to remove all your data from your Mac and return it to factory settings for next person.
A clean install is also something many computer users 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).
Whatever your reason, a clean install of macOS might solve a lot of problems.
However, things have changed in recent years that mean that a clean install may not be necessary, so we will start by explaining why you might not need to do a clean install on your Mac.
You should also keep in mind that there are multiple ways to do a clean installation of macOS. The traditional way was to
make a bootable copy of the macOS installer on a 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). Plus, if you are running macOS Monterey on a M1 Mac or an Intel Mac with a T2 chip then there is a new setting in System Preferences that makes wiping your Mac easier than ever. We’ll run through all of these methods below.
Should I do a clean install on my Mac?
In the past it was helpful to reinstall the system if you wanted to correct some Mac problems, but today this solution doesn’t make as much sense.
Since macOS Big Sur, macOS has its home on its own volume, which is both read-only and cryptographically signed and sealed (referred to as a Sealed System Volume). This seal is stored either in the T2 chip of the newer Intel Macs or in the Secure Enclave of the Apple M1.
Each component of the system is signed in hierarchical order, and any change to a component would also invalidate the seal that represents the top level. For further security, these Macs also do not start directly from the system volume, but from a snapshot of the system. And snapshots cannot be changed, even by the system itself.
So macOS itself cannot be changed by any software that you install as a user. Should a problem occur with macOS during a restart, perhaps because a storage space on the SSD is defective and thus a system component is no longer intact, this will be recognised by the startup process since the seal will now be invalid. Should this happen you will have to reinstall the system.
However, if your Mac boots up without issue then it indicates that there are no problems with the system. As long as your Mac works (which can be determined on the basis of the seal), the Mac is considered to be in working order as it complies with Apple’s specifications.
For that reason, even if you do encounter problems on Macs with T2 chip or Apple Silicon while running macOS Big Sur or macOS Monterey, it makes no sense to reinstall the system yourself.
For those who want to do a clean install so that no leftovers that belong to an old system are carried along, this no longer applies because none of this information is carried over. All components are checked after installation, then signed and finally the entire system sealed again. Again, the seal guarantees that everything is fine and that nothing untoward has crept in.
However, there is one component of the system software that is not installed on the read-only system volume. All other user-installed programs reside on the volume with the user data, including Safari. This means Apple can offer a separate update for the browser and WebKit, so that you do not have to reinstall the whole system each time.
In the case of Safari and other apps there can be benefits in cleaning up your Library folder since it may still contain remnants of old applications you no longer use.
If you feel that you have a lot of this sort of crud on your Mac then cleaning your drive could be useful, just be prepared that it can be a risky business and you may end up deleting something important. Beware that a clean install can cause more problems that it fixes.
If you are happy to take the risk and still want to wipe your Mac and reinstall macOS – which you should definitely do if you are selling or passing on your Mac – read on!
How to wipe your Mac and reinstall macOS
As we explained above, there is more than one way to wipe your Mac and reinstall macOS:
Make a copy of the macOS installer on a USB stick, reformat your drive and then install the macOS on to your Mac from the installer.
Use macOS Recovery to reinstall macOS the web.
Use the new Erase All Content and Settings option setting in System Preferences (available if you are running macOS Monterey on a M1 Mac or an Intel Mac with a T2 chip).
Not only are there three different methods (which we’ll run through below) the methods depend on the version of macOS that you are running.
There is also the even trickier business of installing a clean copy of an older version of macOS on your Mac (Big Sur, Catalina, High Sierra, Sierra, El Capitan, Mojave, Yosemite or an even old version of OS X). Speaking of which, if an old 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 begin…
We recommend that you make 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.