In case you missed it yesterday, Apple announced that macOS 27 Golden Gate will be released on Monday 14 September, and presumably alongside it will be security updates to bring Tahoe to 26.7, and Sequoia to 15.8. What if any preparations do you need to make before upgrading or updating then, or in the coming weeks?

The past

In the old days before macOS Catalina, preparing to upgrade or just update macOS often took several hours. Because all the system and user files were in the same volume, and installers weren’t always too particular about what happened, you had to be prepared for almost anything to occur.

When booting from HFS+ in particular, the chance that a minor file system error could turn into something nastier was significant. So running First Aid over at least the Macintosh HD volume was important if not essential. Occasionally, the updated system couldn’t even boot fully, so it was also good planning to clone the whole boot volume to an empty hard disk. If the updated system failed, then you could always boot from that clone and return to where you were before updating. In addition to that, it was essential to have a full backup completed just before the update, in case anything went missing.

Major changes

The first question to be tackled when preparing for any upgrade is whether this changes something fundamental, as when going from Mojave to Catalina with the removal of support for 32-bit code. While Golden Gate will protest louder over running Intel-only code, it still has full support for Rosetta 2. Other deprecations and removals should affect fewer users, so for the great majority of Apple silicon Macs there shouldn’t be anything major to prepare for.

Next I’ll consider what the upgrade to Golden Gate is likely to involve.

Upgrades

All recent macOS upgrades have been performed as if they were updates, rather than downloading a standalone macOS Installer app to perform a full upgrade, and I expect Apple to follow that on Monday. The components to be updated are therefore:

• mBoot firmware, stored in a hidden container on the internal SSD;
• the Signed System Volume (SSV), a sealed snapshot of the System volume;
• paired Recovery, in the Recovery volume of the boot volume group;
• fallback Recovery, in a hidden container on the internal SSD;
• cryptexes, sealed disk images containing Safari and its supporting frameworks, and large dyld caches for frameworks, stored in the Preboot volume of the boot volume group;
• a few small directories totalling less than 200 MB stored on the Data volume and firmlinked into the SSV.

Apart from the last of those, each component not only has its integrity checked by hashes, but those hashes are validated against Apple’s requirements by their signature hash. Even a single-bit discrepancy in their contents would be detected, and result in failure of the upgrade. That integrity checking replaces most of the checks we performed in the past.

The only volume that could be repaired is the Data volume, and that contains a tiny fraction of the updated macOS. When FileVault is enabled, access to the encrypted Data volume isn’t granted until the user logs back in at the end of the process.

Rescue disk

One common practice in the past has been to keep a bootable external disk ready for use as an emergency rescue, such as a failed upgrade or update. These were most useful when they contained additional utilities such as Alsoft’s DiskWarrior that could perform repairs that weren’t possible using macOS apps, or when they allowed manual adjustments to be made to coax the system to behave.

To a degree, Intel Macs could still do this, provided that it has already been enabled in Startup Security Utility for those with T2 chips. However, doing so allows anyone to start that Mac up from a suitable bootable disk, so reducing its security.

Apple silicon Macs can’t start up fully from an external disk, as they must always begin their boot process from their internal SSD. If that’s capable of getting them going to the point where they could complete booting from an external disk, then it’s most likely they’ll be able to start up in either of their Recovery modes. Apple intends those should replace any need for a rescue disk, and deliver better boot security. As I don’t know of any third-party product I’d find useful on an emergency rescue disk, it’s hard to justify all the work involved in maintaining them when paired or fallback Recovery can do whatever could be useful.

Back up

The one essential preparation for all Macs before they’re updated or upgraded is making a full backup, capable of migrating those files in the Data volume, and any custom volumes you have added to the internal SSD. Ask yourself how you would cope if your Mac’s internal SSD had to be restored in DFU mode. If you’re prepared for that contingency, then your Mac should be ready to upgrade or update.

Third-party kernel extensions

Few Apple silicon Macs should still require the use of third-party kernel extensions, the major exception being that to support SoftRAID, which is now bundled in macOS. If your Mac does require other kernel extensions, you will probably want to delay upgrading to Golden Gate until their vendors have confirmed they’re compatible, or provided updates that are.

Limited free space

The few problems that occur seem largely confined to Macs with limited free space in the container holding their boot volume group before starting the upgrade. Although the Update Brain performing an update makes repeated checks to ensure there’s sufficient space, and runs deleted to recover as much purgeable space as possible during the update, you might move whatever you can from the Data volume to external storage to increase the safety margin before starting the update.

Key points

• Most of the preparations we made in the past are no longer necessary.
• A good recent backup is essential.
• Don’t rely on an emergency disk, but familiarise yourself with Recovery modes.
• Confirm the compatibility of any third-party kernel extensions.
• Allow ample free space, moving files temporarily if necessary.
• Back up before upgrading, sufficient to be able to migrate to a restored internal SSD.

I wish you a happy weekend getting ready!