Next month (October), we should all be able to upgrade to macOS 11 Big Sur. If you’ve decided that you’re going to move at least one of your Macs up to this new major release, you’ll need to start preparing now. Here are some pointers based on my experience with previous upgrades, and what’s changing in Big Sur.

These are primarily aimed at those upgrading just one step from Catalina. Don’t forget that Big Sur builds on the major changes of macOS 10.15. If you’re planning to upgrade from Mojave or earlier, you should also read this article about upgrading to Catalina, and this one about residual problems in Catalina.

Time Machine and backups

Apple has announced, and has been testing in betas, the most substantial change to Time Machine for many years, which enables it to make backups to APFS volumes. It has also been fixing at least some of the bugs in the normal backups made to HFS+ volumes. Although the latter should remain backwards compatible with previous versions of Time Machine, backups on APFS will at best have limited compatibility.

Before any upgrade, regardless of whether you intend persevering with the existing system to HFS+ or switching to APFS, you must not only ensure that you have a full backup using Time Machine, if you use it, but should also make a thorough backup using an independent product such as Carbon Copy Cloner, SuperDuper!, ChronoSync, or similar.

I also strongly recommend that, whether using HFS+ or APFS to store your Time Machine backups in future, you start making a fresh backup set with Big Sur. You’ll have to do that if you switch to using APFS anyway, but following the problems which occurred with Catalina, you’ll be much better off if you archive your old backup set and start afresh.

Whatever you do, don’t assume that earlier versions of macOS will be able to access Big Sur’s backups. If you want to access more recent backups from an older macOS, then you’ll probably find a third party product more reliable.

One important question with backups to APFS is whether you can expect reasonable performance when the backup storage is a hard disk. I don’t know of any evidence on this, but in theory at least the fragmentation problems normally experienced when a hard disk is formatted in APFS should be less likely. However, performance of APFS will always be better on an SSD.

Experience running new Time Machine backups to NAS appears very limited so far, and I regret that I can’t offer any meaningful advice if that’s what you intend to do, other than to go carefully and have a fallback in case it doesn’t work well.

One remaining issue to keep an eye on is whether any cloning or backup tools will be able to clone or restore the new Sealed System Volume (SSV). There are only two methods of doing that: Apple’s system software installers, and asr. Even in late betas, asr hasn’t been able to do this successfully, requiring the SSV to be installed rather than restored, as I’ve explained here.

Scripts and paths

If you’re upgrading from Catalina, you might presume that, as there aren’t any significant changes in volume and folder layout, you don’t need to check the use of paths in scripts and other code. It’s still worth referring to my charts showing Big Sur’s boot volume layout, and testing any scripts which are dependent on fixed paths, or on having access to anything on the SSV.

During beta-testing, some well-known paths and data files haven’t worked. For instance, if you have a script or tool which accesses Installer receipts (BoM files) at /System/Library/Receipts, these haven’t existed in betas and may also be missing in release versions of Big Sur.

If your scripts perform any checking of the current version of macOS, then you’ll need to read this article to understand whether they need to be changed, or you can rely on setting the environment variable to make it appear that Big Sur is actually macOS 10.16.

Secondary updates

When you’re planning for the upgrade, don’t forget that most non-bundled Apple apps are likely to be updated immediately afterwards. These could involve very large downloads, particularly if you use Pro apps or Xcode – and in my case, all of the above.

If you’re upgrading from Mojave or earlier, these may remove 32-bit support for conversion from old movie and other formats. If you’re unsure whether that will prevent access to some important documents, now is the time to perform final checks.

It’s always wise to check that all of your key apps are fully up to date before upgrading. Big Sur (and Apple Silicon) bring some subtle changes which can throw strange problems in apps which you mightn’t expect. I’ve already looked at some of those in this overview.

If you’re unsure whether the current version of an app or other software is going to work properly, dropping it onto my free utility ArchiChect can give a good idea whether it’s a bit old and creaky and could prove a problem.

32-bit software

If you’ve already upgraded to Catalina, then you’ll have worked carefully through all the issues which arise from loss of 32-bit software. If you’re making the leap from an earlier version of macOS, then you still need to undertake thorough checks, such as those explained here. You should find ArchiChect a help here.

If you need to retain access to older apps which won’t run properly, or at all, in Big Sur, then you’ll need to prepare your Mac so that you can do so. In practice, this probably means installing Mojave or an earlier version of macOS into a Virtual Machine, something which should be straightforward if you do it now, using Parallels Desktop, VMware Fusion, or similar.

This has been made more complex because of a major problem with expired certificates on Apple’s system installers and updaters provided before October 2019. If you have an old Mojave installer from before that date, for example, it’s likely that Apple’s intermediate and user certificates on it will have expired on 24 October 2019. You can check this using methods explained here, or with Taccy or ArchiChect.

Access to older versions of macOS is now best achieved through Apple’s support articles. For example, although you may not find it yourself in the App Store, Apple provides a link in this article which opens the Mojave installer page in the App Store for you. That installer appears to fail when you try to run it, and probably needs to be set up on external storage as a bootable installer. But it works perfectly well for a VM.

During beta testing, trying to dual-boot between Big Sur and Catalina has resulted in various odd problems which largely seem to reflect the significant changes in APFS, for example those required to support the SSV. Dual booting with an even wider gap between the installed versions of macOS, to enable the use of 32-bit apps, is unlikely to prove trouble-free. If you can, you should find a VM a much better solution.

Reminder list

As you’re working through these and other checks, build yourself a list of the tasks which you need to complete before and shortly after upgrading. If you’re going to be upgrading from Mojave or earlier, this is going to be a more complex upgrade than normal, but it shouldn’t prove overdemanding for the great majority of users.

Finally, in case you’ve lost the link, here are details of the compatibility of my free apps with Big Sur.