In the past, many of us took the opportunity when upgrading to a major new version of macOS to perform a clean install onto a freshly-formatted disk. In the days when system and user files were all intermingled in the same volume, there were good arguments that the result was worth the extra effort, both in cleaning out the crud accumulated over the years, and in ensuring that we started with a perfect, mint copy of macOS. This article asks what benefits there are to doing this with Monterey or Big Sur, and how you can do it.

Big Sur and Monterey are different from all previous versions of macOS in that the great bulk of macOS is loaded and run from a Sealed System Volume (SSV). That is first installed on the separate System volume, every file checked for integrity, the whole volume ‘sealed’ with a cryptographic hash, and then saved as an immutable snapshot. Every time that system is booted, its integrity is verified against Apple’s master Seal. So as far as the System volume is concerned, there’s nothing to be gained in a clean over a regular install.

Where there can be benefits are in those system files, including Safari, which are stored not on the SSV, but on your writeable Data volume, and in the others there. Those include your bulging Library folder which is bound to contain the remains of all sorts of apps long since removed. If you see this as a good opportunity to clean those up, and are prepared to spend the time doing so, you may find a clean install worth the extra effort.

It doesn’t come without risk, though. Whether you migrate user accounts and apps from a copy or backup, or laboriously reinstall everything you need on your new system, there’s a chance that something important will get left out and cause problems. Sometimes, the migration itself can cause unexpected problems. One example is when you depend on old hardware such as an external RAID drive which is only supported by an old kernel extension. Security policy has changed substantially in recent years, and unsigned kernel extensions can’t readily be installed afresh, although they can still be grandfathered across in upgrades. If an essential kernel extension isn’t migrated to your new system, you could find it almost impossible to install yourself.

If you’re unsure whether anything important will survive migration or reinstallation, a clean install could have serious impact on your Mac, and you may be wiser simply to upgrade in the normal way.

Can you do it all in Recovery mode?

In theory, a clean update should be a perfect task to perform in Recovery mode, but in reality the limited choice of macOS installations may not match your plans. This has also become far more complicated than it should be.

Intel Macs can be booted with Command-Option-R held to enter Recovery mode and be offered the latest available version of macOS which is compatible with that Mac. Before the release of Monterey, that will be the latest version of Big Sur, but once the current macOS version becomes 12, you won’t be able to obtain Big Sur in Recovery mode except through the command line.

M1 Macs don’t currently have an option to enter Recovery mode for the latest version of macOS on offer. Unless Apple changes this on the release of Monterey, the only version Recovery will offer matches that of the installed firmware, although that may not be the current version installed on that disk! For example, when Monterey is released, Recovery should continue to offer to reinstall Big Sur unless you’ve installed one of the Monterey beta-releases on that Mac (even on an external disk), in which case it should offer Monterey. I think.

If you’re confused, you can always boot into Recovery and check, or just give up and opt to use an external bootable disk instead. Unlike installing in Recovery, that also ensures you have a copy of the installer to use again should you need.

The most straightforward way to perform a clean install is to erase your current startup system and install the new system from a bootable installer disk. There are other ways, for example you can restore M1 Macs in DFU mode with an IPSW image of the new version of macOS. You can also run Disk Utility and the macOS installer when booted from a different disk, although that isn’t recommended for M1 Macs unless you’re upgrading another external disk.

Prepare the bootable disk

Connect the flash or other disk you intend to turn into your bootable installer disk, open Disk Utility and format that external disk in Mac OS Extended (Journalled), HFS+J – not in APFS. That may seem strange, but all macOS bootable installers adopt the older file system and are likely to fail if run from APFS.

With the external disk still mounted, and the Big Sur or Monterey installer app available, here for example in your current /Applications folder, open Terminal and type a command of the form
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
for Big Sur, or
sudo /Applications/Install\ macOS\ Monterey.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
for Monterey. Confirm the latter by checking the name of the installer app, and in both cases use the correct path to the installer app, and to the external disk, which is here assumed to be named MyVolume.

The upgrade installer will then be installed on your external disk, to turn it into a bootable installer with a different name. Once that’s complete, you can proceed to erase the existing volume group and perform the upgrade.

Check T2 Macs can boot from an external disk

If you’re using an Intel Mac with a T2 chip, you next need to check that your Mac is configured so that it can boot from an external disk, unless of course you already know that because that’s what it’s doing now. To do that, restart in Recovery mode (Command-R), open Startup Security Utility and enable your Mac to boot from external media. If you don’t do that and that’s disabled, you won’t be able to boot your Mac from the external bootable installer you’ve just made. You don’t and can’t do that on an M1 Mac: this only applies to Intel Macs with T2 chips.

Ensure your migration source is connected

If you’re going to migrate your files from a backup or copy, next ensure that is connected to the Mac, so that it’s available when its time comes during the installation. If you’re going to migrate manually, or simply want to defer migration for the moment, then this isn’t necessary.

Boot from your external installer disk

On an Intel Mac, restart with the Option key held to see the choice of startup disks. Select your bootable installer, and press Return to allow startup to complete.

On an M1 Mac, start up with the Power button pressed until you see that your Mac is loading Options. Wait until all the available boot disks are displayed, select your bootable installer and click Continue underneath it.

Erase the old System Volume Group

Once the installer is running, select the Disk Utility option. Then select the volume group on the disk you want to upgrade. If you have more than one bootable system available, check carefully that you have selected the right one. Click the Erase tool, and in the dialog opt to erase the Volume Group, if that’s offered. If it isn’t, ensure that you erase the Data volume in the group; the System volume will be upgraded anyway.

Install the new macOS

Once the Volume Group has been erased, return to the main installer and complete installation of your new version of macOS. Ensure that this installs to the correct disk, and when the process reaches the migration step, perform any migration you intend doing.

References

How to create a bootable installer
How to erase an Intel Mac
How to erase an M1 Mac
Dealing with M1 personalisation errors when reinstalling macOS