How to clean install macOS on your Mac

Dan October 20, 2022

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 or later on a M1 or M2 Mac, or an Intel Mac with a T2 chip, then there is a 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/M2.

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 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.

You might prefer to use an application like CleanMyMac X from MacPaw ($39.95/£34.95 a year, free trial), or one of these alternatives: Best Mac Cleaner App.

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!

Three ways to clean install macOS

As we explained above, there is more than one way to wipe your Mac and reinstall macOS:

  1. 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.
  2. Use macOS Recovery to reinstall macOS the web.
  3. Use the Erase All Content and Settings option setting in System Preferences (available if you are running macOS Monterey and later on a M1 or M2 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 (Monterey, 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.

Erase Mac Monterey

Before you begin: Back up your 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.

Option 1: Erase content and settings (macOS Monterey and later)

Delete M1 M2 Mac

Foundry

Before the arrival of Monterey it was necessary to create a bootable USB drive or use Recovery Mode to wipe your disk. However, following Monterey Apple has made it easy to get a factory-fresh installation.

There is a new Erase All Content and Settings option in the System Preferences menu of Monterey that will allow you to reset your Mac to factory settings without having to work through the many steps to wipe and format your drives.

As well as macOS Monterey, you need to have a M1 or later Mac, or an Intel Mac with a T2 chip inside.

These Intel Macs have the T2 security chip:

  • MacBook Air from 2018
  • MacBook Pro from 2018
  • Mac mini from 2018
  • iMac 27-inch from 2020
  • iMac Pro
  • Mac Pro from 2019

If you have the Erase All Content and Settings option in Monterey then you can easily wipe your system, remove all your data and user-installed applications and start from scratch. Here’s what you need to do:

  1. Open System Preferences.
  2. Click on System Preferences in the menu bar at the top of your screen.
  3. Click on Erase All Content and Settings. 
  4. Enter your password.
  5. (You’ll be reminded to do a backup with Time Machine before you begin deletion – but you can skip that step.)
  6. A window will show you what will be deleted, including Apple ID, Touch ID, accessories, and Find My settings.
  7. Finally you will need to log out of your Mac and all related services using your Apple ID.
  8. Before your Mac is erased you receive a final warning, so if you have second thoughts at least you can stop at this point!
  9. Having clicked the second Erase All Content & Settings your Mac will restart. Eventually you will see a black screen while it restarts.
  10. Once your Mac restarts you can go through the steps to set it up as new, or you can leave it at this stage for the next owner to set up. Just turn the Mac off first.

Option 2: Do a clean install via Recovery

Install macOS via Recovery

Foundry

We explain how to reinstall macOS via Recovery here: How to reinstall macOS, but we will summarise the steps here.

How you enter recovery mode depends on the Mac you own.

If you want to enter recovery mode on an M1 or M2 Mac you need to press and hold the power button when you start up your Mac and continue to press and hold the button until you see a startup options window appear with an Options icon. Select Options and click Continue.

To enter Recovery on an Intel Mac press Command+R.

  1. Once Recovery starts up you’ll see a macOS Utilities window with options to Restore From Time Machine Backup, Reinstall macOS [version], Safari (or Get Help Online in older versions) and Disk Utility. If you want to wipe the Mac before reinstalling choose Disk Utility. 
  2. Run through the steps to erase your Mac as detailed in the How to reinstall macOS guide.
  3. After you have erased your drive click on Disk Utility > Quit Disk Utility. Or just Command + Q.
  4. Now choose Reinstall macOS from the options.
  5. Wait while macOS is reinstalled. Expect this to take a while.

Option 3: Do a clean install macOS using a bootable USB

How to clean install macOS using a bootable USB

Now for the method that involves using a bootable USB with the macOS installation on it. This might be the best method if you already have the installation files downloaded and don’t want to wait for hours for Recovery to download the files.

At a glance
  • Tools required: 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.
1.

Format the external drive

Erase the drive

Foundry

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.
2.

Get the macOS install file

Find installer macOS

Foundry

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 Monterey and want to do a clean install of macOS Ventura then you can download the installation package via System Preferences.

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

If, on the other hand, you want to reinstall Monterey and are already running Monterey the best way to access the installer for Monterey is via the Mac App Store. Here’s a link to Monterey on the Mac App Store (or Big Sur or Catalina). Alternatively open Mac App Store and search for the version you want.

Once you have located the version of macOS you want 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 start the installation!
  2. Press Command + Q to quit out of the installer.
  3. Find the installer file (it will be located in your Applications folder, or you could search for it using Spotlight (command + Space and start typing Ventura, Monterey, Catalina or Big Sur).

If you just want to install Ventura you can skip the next step where we’ll be showing how to dig up the installer file for an older version of macOS or Mac OS X.

3.

Get an install file for an older version of macOS

Download High Sierra

Foundry

For older versions of macOS and Mac OS X it’s a bit more difficult to get the install file: Apple doesn’t offer downloads of outdated OS versions through the normal store front, and searching for the name won’t find what you’re looking for.

We have a detailed guide to getting older versions of macOS here, but we will explain what you need to do below.

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.

Log into the Mac App Store with the Apple ID you used and click Purchased in the menu bar along the top. Scan down the list of apps you’ve purchased or download for free (which is in order of when you downloaded them) and find the name of your chosen version: El Capitan, Yosemite or whatever. Click download.

However, there are a few reasons why this may not work for you:

  • If you have already installed Mojave, Catalina or Big Sur on your Mac you cannot easily search for old 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.
  • And you won’t find versions of macOS in the purchased section if you never installed that version.

However, luckily it is still possible to get hold of some versions of macOS if you are running a newer version than Sierra.

  • Big Sur is on the Mac App Store here.
  • Catalina is on the Mac App Store here.
  • You can get macOS Mojave via this link.
  • Download High Sierra here.
  • Sierra is here as a direct download.
  • El Capitan can be downloaded from this link.
  • Yosemite is available here. 

If you click to find Catalina, Mjave or High Sierra and the page doesn’t open in the Mac App Store try a couple of things: Make sure that the Mac App Store is closed before you click the link; Cut and paste the link into Safari.

As in the step above, once you have downloaded the installer don’t run it – just close the installer if it opens.

For more advice on getting hold of even older editions of Mac OS, see How to get old versions of OS X.

4.

Create a bootable USB disk

Createinstallmedia mac

Foundry

Next we need to turn out USB drive into a bootable USB disk so that we can use it to install a new copy of macOS on our Mac after we have reformatted it (that step will come later in this tutorial).

We’ve got full instructions on How to make a bootable macOS install drive in a different article, but we’ll run through the basics here.

We’re going to create the bootable drive using Terminal, which you can open on your Mac by pressing Command + Space bar and typing Terminal.

Connect the USB drive that you formatted earlier to your Mac. Make sure it’s called Untitled – rename it if necessary (this will help you enter the correct text in Terminal in the next steps).

Creating a bootable installer has been pretty easy since the arrival of Mavericks in 2013 thanks to the Terminal command createinstallmedia that arrived in that version. Here’s how to use it (note that the name of the version of macOS will depend on which version you are installing):

  1. Open the Terminal application on your Mac (find it by searching using Command + Space and typing Terminal)
  2. Now copy and paste the following text into Terminal (This text is for Big Sur – we include the text needed for the other versions of macOS in this article). When you cut and paste the following text into Terminal do check that it maintains the — as we have heard of people pasting in the — and it converting to –)
    sudo /Applications/Install\ macOS\ Monterey.app/Contents/Resources/createinstallmedia –volume /Volumes/MyVolume (where MyVolume is the name of the external drive you are using – it could be Untitled)
  3. Press Return.
  4. Enter your password (you won’t see the password appear).
  5. Press Return again.
  6. Terminal will warn that it is about to erase the drive. If you want to continue press Y and then Return. The process can take a while, you’ll see “Erasing Disk: 0%… 10%… 20%… 30%…100%…
  7. Next Terminal will spend a few minutes copying the installer file to your drive. “Copying installer files to disk… Copy complete” and so on will appear in the Terminal window. Again, this can take a while – it took our Mac ages to go from 0% to 10%…
  8. When the bootable drive is prepared you will see the word Done. It can take a while.

Now you have the installer on the external drive you can use that to install copies of macOS on multiple Macs – which could be helpful if you are admin for a lot of systems.

However, if you are reading this because you want to do wipe your Mac and do a clean install, read on…

5.

Wipe your Mac

Wipe your Mac - view in Disk Utilities

Foundry

Now we are ready to install the copy of macOS from the installer drive on our Mac. If you want to do a fresh installation (and completely wipe your Mac) it’s time to reformat your Mac’s start-up drive.

If you are running macOS Catalina, Big Sur or Monterey you will have two volumes: Macintosh HD and Macintosh HD – Data. If you want to do a clean install of macOS over an existing installation you will need to first delete the Macintosh HD – Data volume. We cover how to do this in more detail here: How to reset and wipe a Mac.

Macintosh HD – Data is where your data is stored separately to the macOS installation.

  1. Either click on the – button or go to the menu and choose Edit > Delete APFS volume.
  2. You will see a message warning you that this will permanently erase your data. Click on Delete. Do not choose Delete Volume Group.
  3. Wait while the volume is deleted.

Then you can move onto deleting your Macintosh HD volume as above.

Here’s how to do a fresh installation of macOS if you aren’t already running Catalina, Big Sur, Monterey or later.

  1. Connect your boot drive.
  2. Start up – or restart – your Mac while holding down the Option key (also known as Alt). This will take you to Startup Manager.
  3. Choose to install your chosen version of macOS from the external drive.
  4. Select Disk Utility.
  5. Select your Mac’s start up disc, probably called Macintosh HD or Home.
  6. Click on Erase.
  7. Wait for your startup disk to be formatted.
  8. Go back to the main Utilities menu and choose Install macOS.
  9. Choose your startup drive as the location to install macOS.

Once installation is complete, you can restore apps and settings from your backup, or download them again manually.

6.

Install macOS

macOS 13 ventura logo

IDG

Now you can use your bootable installer to do a clean install of macOS on your wiped Mac.

The process for installing the OS from the bootable installer is slightly different if you have an M1 or M2-series Mac (also known as Apple Silicon) or an Intel processor inside your Mac. We’ll run through both methods.

In both cases you will need to have your Mac connected to the Internet. It also needs to be compatible with the version of macOS you are trying to install (you can’t install a macOS older than Big Sur on a M1 Mac).

How to install macOS from a bootable installer

If you have an M1 or M2 Mac here’s how to install macOS from a bootable installer. As we mentioned above, you can’t install a macOS older than Big Sur on a Apple Silicon Mac.

  1. Plug in your bootable installer
  2. Turn off your Mac.
  3. Press the power button to turn on the Mac – but keep it pressed until you see the startup options window including your bootable volume.
  4. Choose the bootable installer volume.
  5. Click Continue.
  6. The macOS installer should open – when it does follow the onscreen instructions to do the clean install.

If you have an Intel-powered Mac here’s how to install macOS from a bootable installer:

  1. Plug in your bootable installer
  2. Turn off your Mac.
  3. Press and hold Option/Alt while the Mac starts up – keep pressing the key until you see a screen showing the bootable volume.
  4. Choose the bootable installer volume.
  5. Click Continue.
  6. Select Install macOS from the Utilities window that appears. 
  7. Click Continue.
  8. Now follow the onscreen instructions to do your clean install.

Mac, Personal Software

Read original post from Macworld.

Categories: Macworld
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