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. 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.

Everything in Catalina and more

If you’re not already running Catalina, then Big Sur will come as a big surprise: it’s everything in Catalina and more. 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 familiar with the changes which come with Catalina, you may find this article a useful refresher.

The Sealed System Volume

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. Your Mac boots not from the System volume, but from that sealed snapshot.

This has pervasive effects. Although you can unseal the System, that defeats one of the major purposes of Big Sur. So what Apple puts on the System volume, in the way of all its standard fonts, for example, is what you’ve got. 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. 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.

This is the structure of a Mojave boot disk:

And this is what you get in Big Sur:

This has had significant effects on macOS installers for updates. 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. Apple has therefore not provided standalone updater packages for Big Sur, and doesn’t appear to have any intention of doing so in the future. The only type of standalone updater available is the full installer app, at around 12 GB.

We don’t know yet how this will work out once Monterey is released and Big Sur receives only security updates. I suspect that those updates will be significantly larger than those released for Mojave or Catalina, and their only alternative will be full installer apps, but maybe Apple will surprise us with something more flexible.

Because the Sealed System Volume behaves more like firmware, this has also had effects on backing up and cloning. Intel Macs can still just about clone Big Sur boot volume groups which work properly, but this is no longer recommended. Neither can Time Machine back your System volume up. Although this may appear a devastating change, in reality it has significant advantages. Instead of relying on a cloned System, Big Sur much prefers you to install macOS afresh in Recovery, and will, given a suitable Data volume, link the fresh System volume with that, or perform an efficient migration from Time Machine and other backups.

You may find this article helpful.

Time Machine backups

Apple curiously downplayed one of the biggest new features in Big Sur: making Time Machine backups to APFS storage (TMA). Traditional Time Machine backing up to HFS+ was slowly grinding to a halt in Catalina anyway, and many improvements were required to restore it to its former glory. Backing up to an APFS volume is quicker, more economical on storage space, and far more reliable.

The snag is that TMA backups are very different to Catalina’s, so to take advantage of TMA’s features you’ll have to start a new backup set. You’ll also discover that if you do start a new backup set, it must use TMA and not back up to HFS+. You’ll therefore need to put your backups at the centre of your upgrade plan.

One effective solution, if you can afford it, is to make one final backup in Catalina, unmount your backup storage and put it away safely. Then attach new storage and make a copy of your existing boot disk to it, using Carbon Copy Cloner or SuperDuper! Once that’s complete, upgrade, and set your new backup storage as the destination for Big Sur TMA.

I have written a whole series of articles about TMA, including:
Time Machine to APFS: Evolution
Time Machine to APFS: Initiating an auto backup
Time Machine to APFS: Understanding backups
Time Machine to APFS: Backing up
Time Machine to APFS: How processes have changed
Time Machine to APFS: Backup structure and access
Time Machine to APFS: Maintenance and repair
Time Machine to APFS: Changing disks
Time Machine to APFS: How efficient are backups?
Time Machine to APFS: First full backup
Time Machine to APFS: Using a network share
Should you back up to APFS or HFS+?

User interface

Many first reactions to Big Sur’s new user interface are of horror. If you’re still unsure about it, there’s no shortage of examples of the changes that it brings. In practice, I find it fine to use all day, in both Light and Dark Modes, although I think it works better for those who prefer Dark Mode. If you rely on some of the visual cues and aids in Mojave, you may find Big Sur more of a struggle, particularly in window controls, where there are remaining annoyances which don’t improve with practice, I’m afraid.

Remaining bugs

In addition to those I’ve mentioned above, Big Sur does still have plenty of annoyances, features that just don’t work properly. One of my pet hates is the Bluetooth menu bar tool, which only ever gives correct charge levels for your keyboard and mouse/trackpad on the second time that you open it to check.

Those who use Apple’s Mail app will be delighted to discover that its column layout is much improved over that in Catalina, but there remain questions over Mail’s longstanding bug(s) which can result in mailbox corruption. These now seem to affect fewer users, but it’s no consolation if you turn out to be one of those few who do have problems.

Is it safe? It’s certainly a lot safer than the early releases of Big Sur were, and provided that you can live with its SSV and interface, I think it’s a great improvement on Catalina. Early indications are that Monterey will bring fewer changes and consolidate more, so as a stepping stone to macOS 12 it’s a good upgrade. I wish you a smooth and trouble-free upgrade, and happiness with Big Sur.