Until July 2011, with the release of Mac OS X 10.7 Lion, Macs didn’t have any Recovery Mode. I suppose its nearest equivalent was single-user mode (SUM), which for many was the preferred way of running fsck to check and repair your startup disk, and didn’t we need to then. When Apple added Recovery Mode, it was kept on a hidden HFS+ volume, and the layout of boot disks started to become more complex. This article is an atlas of boot disks from the next step with High Sierra and APFS, to the present in Monterey, with particular reference to Recovery volumes.

Throughout these diagrams, Unix-style identifiers are only examples of what you might see, and will vary in use. For example, a Preboot volume shown here as having an identifier of disk7s3 could be anything from disk7s1 to disk7s7 instead, and when in the container disk3 would be anything from disk3s1 to disk3s7.

High Sierra and Mojave

A standard APFS boot disk in High Sierra and Mojave consists of an EFI partition and the main APFS container. Within the latter are a minimum of four APFS volumes:

• the startup volume, by default on the internal disk named Macintosh HD, which forms the root of the boot file system,
• Preboot, a small volume of around 40-50 MB,
• Recovery, the Recovery Volume, of around 500 MB,
• VM, containing virtual memory caches, which is upwards of 20 KB depending on use.

Because some boot disks, notably Fusion Drives, don’t support APFS in High Sierra, some continue to use HFS+, in which case those are instead HFS+ volumes, which can’t be inside a container.

Each boot volume group has only one Recovery Volume, booted by holding the Command and R keys during startup. If that fails, the only alternative source of recovery tools is Remote or Internet Recovery, entered with the Command, Option and R keys held.

Catalina

Catalina changes that by splitting the startup volume into two:

• the read-only System volume, by default on the internal disk named Macintosh HD, which forms the root of the boot file system,
• the writable Data volume, by default on the internal disk named Macintosh HD – Data, which is normally hidden from view at /System/Volumes and accessed via firmlinks,
• Preboot, a small volume of around 40-50 MB,
• Recovery, the Recovery Volume, of around 500 MB,
• VM, containing virtual memory caches, which is upwards of 20 KB depending on use.

The big difference in the volume layout is that disk1s1 and disk1s2 form a Volume Group, within the bootable container disk1 on your boot disk disk0. Otherwise the layout is the same, and this only applies to bootable APFS containers, not to non-bootable APFS containers on regular storage. With its single Recovery Volume, the same startup key combinations apply.

Big Sur

Big Sur brought changes to accommodate the new Recovery System on M1 Macs. On those, but not on Intel models, the Recovery System moved into its own APFS container (if you must, partition). Boot an Intel Mac into Recovery in Big Sur, and it boots from the Recovery Volume in its main APFS container; boot an M1 Mac into Recovery in Big Sur, and it boots instead from the Recovery container on its internal SSD – and only ever from that internal storage.

Apple silicon Macs actually have two Recovery systems: normal Recovery is entered by holding the Power button pressed during startup until the display reports that Options are being loaded. If that doesn’t work properly, you can start up in Fallback Recovery instead, for which you press the Power button twice in rapid succession, holding the second press in until the display reports that Options are being loaded.

Fallback Recovery is created when a macOS update replaces the currently installed Recovery Volume, and doesn’t exist until that Mac has installed at least one macOS update. Its contents are then copied to the Recovery volume within the current boot volume group. The end result is the latest Recovery system is in the Recovery container of the internal SSD, and its version should match that of the volume group; the fallback Recovery system is stored in the Recovery volume in the boot volume group, and is copied from a previous version of macOS. This means that Internet Recovery is no longer needed.

Intel Macs, though, don’t have the same container structure, or any Recovery container boot. They still boot into the Recovery volume within their boot volume group by holding the Command and R keys during startup. If that fails, the only alternative source of recovery tools is Remote or Internet Recovery, entered with the Command, Option and R keys held.

Monterey

Volume layout on Intel Macs hasn’t changed since Big Sur. There’s the traditional hidden EFI partition, and a single APFS container with the bootable system, consisting of:

• the SSV, a mounted snapshot of the unmounted read-only System volume named Macintosh HD, which forms the root of the boot file system. The snapshot is named com.apple.os-update- followed by its UUID, and the volume (hence its snapshot) is typically about 15 GB in size;
• the writable Data volume, by default on the internal disk named Macintosh HD – Data, which is normally hidden from view at /System/Volumes and accessed via firmlinks. On Intel Macs, this is given its full name;
• Preboot, a small volume of around 714 MB;
• Recovery, the Recovery Volume, of around 1.1 GB;
• VM, containing virtual memory caches, which is upwards of 20 KB depending on use.

Note that the Seal on the unmounted read-only System volume is normally broken, but it’s the snapshot which is the important one: that should be sealed, unless you have broken its seal intentionally.

Intel Macs still boot into the Recovery volume within their boot volume group by holding the Command and R keys during startup. If that fails, the only alternative source of recovery tools is Remote or Internet Recovery, entered with the Command, Option and R keys held.

The internal SSD in an Apple silicon Mac (M1 and M2) consists of three APFS containers, and lacks the legacy EFI partition. Only the Apple_APFS container is normally mounted, and that has a similar structure to the boot container of an Intel Mac, or that of an external bootable disk. There are two significant differences, though:

• the Data volume isn’t named Macintosh HD – Data, as on an Intel Mac, but plain Data;
• the Recovery volume is the primary Recovery system for that copy of macOS, and is supported by the fallback Recovery system in the unmounted Apple_APFS_Recovery container, which is larger at 5.4 GB.

Apple silicon Macs are distinct in having this fallback Recovery system which makes them more robust in the event of damage occurring to the boot container on their internal SSD.

Apple silicon Mac hosting Monterey guest in a VM

The standard bundle containing the VM on disk consists of:

• AuxiliaryStorage, a little under 34 MB,
• Disk.img, the main VM block storage, amounting to the designated disk size,
• HardwareModel, a mere 150 bytes,
• MachineIdentifier, about 60 bytes,
• optional copy of RestoreImage.ipsw, the IPSW image installed in the VM, typically around 14 GB.

Different virtualisers may use their own variants, but those above are common to both VirtualBuddy and Viable.

Within the guest macOS its boot disk is seen as having exactly the same layout and structure as the internal SSD of an Apple silicon Mac, with its three containers, and each of the volumes normally found in the boot volume group. These should appear as shown in this diagram:

Entering Recovery Mode in a VM isn’t officially supported by macOS, although it is available in VirtualBuddy. Official support for a VM boot option to start up in Recovery Mode is otherwise only available in Ventura hosts.

Ventura

Indications from beta-releases are that macOS 13 will use the same structures as Monterey, and the same Recovery Volumes. The only slight variation is that its new patch updates will use secure storage. I will publish a detailed account when Ventura has been released.