Although routine security data updates (MRT, XProtect, etc.) are accommodated by storing those few components on the Data volume (firmlinked into the correct path of the System volume), macOS updates and Security Updates require a very different installation process which culminates in the sealing of the System volume and creation of its snapshot. This has had significant effects on macOS installers for updates. This is the structure of a Mojave boot disk: If you use software which needs to make changes at the root level of the System volume, then it can’t: you’ll need to find an alternative route. If you rely on adding your own top-level folders/directories, then you need to be able to do that on the Data volume instead. So what Apple puts on the System volume, in the way of all its standard fonts, for example, is what you’ve got. Although you can unseal the System, that defeats one of the major purposes of Big Sur. Your Mac boots not from the System volume, but from that sealed snapshot. The major achitectural change from Catalina is that Big Sur’s System volume isn’t just mounted read-only, it’s not mounted at all! Instead, what happens during install and updates is that every file on the System volume has a cryptographic hash calculated, those are hashed again up into a tree with a master hash, the Seal, at the top, then a snapshot is made of that. If you’re not familiar with the changes which come with Catalina, you may find this article a useful refresher. It won’t go near anything 32-bit, for which you’ll need to install Mojave in a Virtual Machine, the boot volume is divided into a Volume Group, and its System volume remains for Apple’s use only, not yours. If you’re not already running Catalina, then Big Sur will come as a big surprise: it’s everything in Catalina and more. This article looks at some of the issues you’ll need to address if you’re going to take that step soon, or leave it until Monterey is released in the autumn/fall. With the release of macOS 11.5 just a few days away, many more cautious users are asking whether it’s now safe to consider upgrading.
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