Dual-booting your Mac with multiple versions of APFS

Since its first public release seven years ago in High Sierra, APFS has changed greatly. Connect a bootable external disk with Sonoma installed to a Mac running macOS 10.13 and it won’t know what to make of it. That’s because many of the features used by APFS today didn’t exist until Catalina and Big Sur. This article explains how your Mac can cope with running such different versions of APFS, and how to avoid the problems that can arise when a Mac can start up in two or more different versions of macOS.

Fences

Disk structures, APFS and macOS fall into three main phases:

High Sierra and Mojave, which can run 32-bit code, boot from a single integrated volume, and don’t understand Signed System Volumes (SSVs); they run APFS versions 748.x.x to 945.x.x.
Catalina (macOS 10.15), which can’t run any 32-bit code, and boots from a group of volumes where the system is contained in its own read-only volume; it runs APFS version 1412.x.x.
Big Sur and later, which can’t run any 32-bit code, and boot from an SSV that’s a specially sealed snapshot on the System volume; they run APFS version 1677.x.x and later.

Fuller and finer details of the changes with different versions of macOS are given in the Appendix at the end.

If you need your Mac to run more than one version of macOS, it’s easiest and most compatible if the versions installed come from only one of those phases. Two phases are more of a challenge, and all three needs great care to ensure that one version doesn’t damage the file systems of another.

When you do need to run macOS from two or more phases, the cleanest and safest way is to only mount disks and volumes from one phase at a time. For example, when running Mojave, if you unmount disks and volumes for all later versions of macOS, then you won’t see any notifications warning you of Incompatible Disk. This disk uses features that are not supported on this version of macOS. Unfortunately, unless all your boot systems are on external disks, this isn’t easy to achieve.

Warnings

Generally speaking, APFS tools including those run by Disk Utility (including fsck_apfs, used by First Aid) are backward-compatible, but may not be as reliable when a tool from an older version of macOS is run on a newer version.

First Aid and fsck_apfs normally report versions of APFS tools used on the volumes and containers they’re checking. These draw attention to any potential for incompatibilities, such as:
The volume [volume name] was formatted by diskmanagementd (1412.141.1) and last modified by apfs_kext (945.250.134).
telling you that volume was created by macOS 10.15, and last changed by macOS 10.14.

Other messages you might see include warnings like: warning: container has been mounted by APFS version 2236.141.1, which is newer than 1412.141.3.7.2, which is less helpful.

If you’re going to run multiple versions of macOS on the same Mac, you’ll have to get used to those. For reference, APFS versions decode to macOS versions as:

249.x.x is the beta release from macOS 10.12 Sierra
748.x.x is 10.13 High Sierra
945.x.x is 10.14 Mojave
1412.x.x is 10.15 Catalina
1677.x.x is 11 Big Sur
1933.x.x is 12 Monterey up to 12.2
1934.x.x is 12.3 Monterey and later
2142.x.x is 13 Ventura
2235.x.x is 14 Sonoma up to 14.3
2236.x.x is 14.4 Sonoma and later
2311.x.x and later is macOS 15 Sequoia beta.

Dangers

To minimise the risk of any problems arising, any manipulation of the file system should be performed by Disk Utility, the diskutil command tool or others, for the same or later version of macOS. Thus, if you have Mojave and Big Sur installed, you could use tools from either to maintain Mojave file systems, but should only use those for Big Sur when maintaining Big Sur’s file systems.

This becomes more difficult when file system maintenance needs to be performed in Recovery mode. Once the Mac has started up in Recovery, check the version of macOS before opening Disk Utility, using fsck_apfs, diskutil or anything similar.

This is one situation where versions of macOS with an SSV can become tricky, because of their different associations with Recovery systems. On Apple silicon Macs, Big Sur doesn’t use a paired Recovery volume, whereas Monterey and later do. To be confident that the Mac starts up in the correct version of Recovery, it should have been running that version normally before you start it up in Recovery. Once in Recovery, check the version of macOS before proceeding to work with its file systems.

Appendix: APFS and macOS version details

APFS major version numbers change with major version of macOS:

macOS 10.12 has APFS version 0.3 or 249.x.x, which shouldn’t be used at all.
10.13 has 748.x.x, which doesn’t support Fusion Drives, but has basic support for volume roles.
10.14 has 945.x.x, the first version to support Fusion Drives.
10.15 has 1412.x.x, the first version to support the multi-volume boot group, and introduces extended support for volume roles, including Data, Backup and Prelogin.
11 has 1677.x.x, the first version to support the SSV, and Apple silicon. On M1 Macs, it doesn’t support the paired Recovery volume, though.
12 has 1933.x.x until 12.2, thereafter 1934.x.x, which support the paired Recovery volume on Apple silicon.
13 has 2142.x.x, and is probably the first to support trimming of UDRW disk images and their storage as sparse files.
14 has 2235.x.x, until 14.3, thereafter 2236.x.x.

Minor version numbers increment according to the minor version of macOS, and patch numbers wander without pattern.