Upgrading dos and don’ts…
There may be no benefit in wiping your old system and installing afresh
Check all your key apps are known to be compatible with Monterey before upgrading. Don’t press ahead only to discover that one won’t work for a couple of months yet. This applies particularly to Adobe apps and others whose update cycle lags Apple’s.
Bring those key apps up to date before upgrading. If an app can’t start up properly in Monterey, it’s going to be harder to update later.
Ensure the disk you’re upgrading has ample free space. The installer needs extra room to unpack and prepare macOS 12, and is best given a minimum of 40GB or so to work with.
If you’ve got time, do your housekeeping to remove old apps, duplicates, and unwanted media and other documents before upgrading. This is a perfect opportunity to ditch those movies you’re never going to watch again.
When system and user files shared a single volume, it was better to perform a clean reinstall when upgrading to a new version of macOS. That ensured all the system files were in fine fettle, and any corruption was cleaned up. Changes in startup volumes have now removed any benefits that reinstalling once had in terms of ensuring that macOS is in pristine condition.
Monterey divides your startup disk between several different volumes and the System volume containing only macOS system files, which is sealed to guarantee its integrity. That’s not even used as a regular volume, but is frozen into a snapshot, and your Mac runs its system software from that snapshot, containing special firmlinks to your files, which are stored on the separate Data volume.
To create that snapshot of Monterey’s system, the installer copies the system files onto the System volume and checks each by making a “hash” guaranteeing it’s correct. It next builds a tree of those hashes, culminating in one master hash, the Seal, to guarantee them all. It then makes a snapshot of its new System volume, ejects that volume and mounts the snapshot, which can’t be changed. When your Mac starts up, its Seal is checked against
Apple’s, and you can only log in if they match.
Like Big Sur before it, this guarantees that Monterey can only run from an identical copy of the system. Thus erasing your existing System volume does nothing to make the upgrade any better.
CLEAN REINSTALL
You can still start your Mac up from a bootable installer disk and install a completely fresh startup volume group, but then the effect is more on your own Home folder, Applications and Library folders rather than on macOS itself. It also inevitably increases the risk of losing files.
This is because a clean reinstall also wipes your Mac’s Data volume, which you then need to reconstitute by migrating your apps and Home folder from your backup of the Data volume. Before even considering doing this, you must be absolutely confident that your backups will prove reliable.
If your Mac is an Intel model with a T1 or T2 chip, you’ll need to take a quick trip into Recovery mode before you start. There, open Startup Security Utility and enable your Mac to boot from external media, or it won’t be able to start up from the external installer disk at all. You don’t do that on an M1 Mac, though.
A clean reinstall can be a good way to help remove accumulated dross on a Mac which has been upgraded and migrated along a chain from older Macs. It may be the best approach to clearing out old kernel extensions, and redundant folders.
But it can also create complications as some older extensions may stop working. If you rely on an old RAID storage system, that could prevent you from accessing that peripheral altogether. If you’re not confident it will work, don’t even try it.