Inside macOS installer apps and disks

With the changes brought in the structure of macOS Big Sur, its installers have also changed. This article explains in broad terms how they work, and why they’re different.

From Big Sur onwards, Apple provides several different presentations of macOS installers and updates:

  • Updates are only delivered through Software Update or its command tool equivalent softwareupdate, and have to be downloaded from Apple’s servers, or delivered through a local Content Caching Server.
  • Full macOS installer apps are offered in the App Store then delivered through Software Update.
  • Full macOS installer packages are available through softwareupdate or direct from Apple’s servers, and are named InstallAssistant. When installed, these create a full installer app.
  • Full macOS installers are offered in Recovery, from where they’re downloaded from Apple’s servers. It’s also possible to run a downloaded installer app from Terminal in Recovery, using the startosinstall command (not M1 Macs) or InstallAssistant_springboard.
  • Full IPSW images containing firmware, Recovery and macOS partitions can be installed to restore M1 Macs in DFU mode, using Apple Configurator 2. Those effectively reset that Mac to factory condition with that version of macOS pre-installed. macOS can’t be extracted from these.

Mr Macintosh and various other sites provide comprehensive lists and links to the full installers and IPSW images currently available.

The Installer app

Inside the full installer, the great bulk is a single disk image named SharedSupport. Within that is a folder named com_apple_MobileAsset_MacSoftwareUpdate, and within that is a Zip archive of around 11.5 GB which contains the system to be installed. There are other Zip archives, including one named UpdateBrain, which contains many of pieces required to put it all together.

Essentially what a full installer app does is mount its SharedSupport disk image, unZips from it what it requires as a minimal system for the purposes of the installation, boots from that and gets on with its job of creating the snapshot of the new sealed system.

Among the things which the installer checks is the current version of firmware; if that needs updating, new firmware may be copied to the hidden EFI partition as a staging point for firmware installation to take place. This is particularly complex for Intel Macs with a T2, which shut down the whole of the Intel chipset when installing their iBridge firmware. During that the Mac appears to have shut down completely, as there’s no CPU or display.

Once the new system is in place, sealed and its snapshot created, the Mac boots from that for personalisation.

Before any of this can take place, though, macOS has to check the installer app. These days, if it’s going to install a significantly older version of macOS, the app won’t be allowed to run, and may report odd errors, claiming for example that the installer is damaged. In that case, the only solution is to perform the installation from a bootable installer disk.

Bootable Installer disks

The most robust way to install macOS outside Recovery mode is normally to make an external bootable installer disk using the full installer app. Apple details this here, although it doesn’t explain how this works. There are several odd features of this procedure which merit further examination:

  • The external bootable installer can be on almost any storage medium, including slow USB ‘thumb’ disks, many of which wouldn’t normally be able to host a bootable system.
  • The bootable disk is formatted not in APFS, but still in HFS+.
  • Installer apps for versions of macOS which can’t be run in the current macOS remain functional when run from a bootable installer disk.
  • You normally have to boot your Mac from this disk either through Startup Manager (Intel Macs) or Recovery (M1 Macs).

These are the result of the bootable installer disk being very different from a normal macOS boot disk. Instead, it’s fundamentally a convenient way of delivering the installer in a disk image. Booting from a bootable installer mounts a disk image containing a minimal system for the purposes of the installation, which the Mac then boots from and uses to install macOS. For M1 Macs in particular this process is far simpler than the Secure Boot required for a full external bootable macOS system, which explains why Flash disks make excellent bootable Installer disks although they may not be able to host a full bootable macOS.