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If you plug an external macOS system disk using APFS into a newer version of macOS you will get a warning afterwards back in your original macOS saying: 

"Incompatible Disk. This disk uses features that are not supported on this version of MacOS"

How to reproduce: Install mojave to an external drive. When everything runs fine shutdown. Take the disk and mount it under a system running big sur. Eject the disk. Boot mojave again from the external drive and see the above error.

You don't need to do anything with that disk in the newer OS, mounting and unmounting again is sufficient. It does only happen to disks containing a macOS system.

I had that issue with several macOS combinations:
mojave disk -> mounting under big sur -> boot back to mojave
high sierra disk -> mounting under big sur -> boot back to high sierra

Finding a solution I only read the advice to erase the whole disk and reinstall.

Can anyone tell what APFS or container feature(s) macOS adds and if it is possible to remove that feature(s) with a tool like diskutil or similar without erasing?

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2 Answers 2

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The newer versions of macOS have the root volume composed from a read-only volume and a read/write volume. The older versions of macOS have not been patched with software to correct interpret this type of configuration. Instead, these older versions have at least been patched to recognize the newer configuration exists and to display the warning message posted in the OP's question.

I have a 2018 Mac mini with Mojave installed on the internal drive. The external Thunderbolt 3 drive has both Mojave and Big Sur installed in the same APFS container. Furthermore, internal Mojave has the home directory for each account manually set to the home directory of the corresponding account on the external Mojave volume. (The external Mojave has the home directories in the default location.) When booting the internal Mojave, the warning message is displayed. This configure has been in use since Big Sur was released and no one has noticed any problems.

In other words, read and writing† to a APFS volume in a APFS container with a newer version of macOS apparently does not create problems. However, you would not want to use an older version of macOS to verify or repair an APFS container with a newer version of macOS installed.

IMO, if I was to remove Big Sur from the external drive, then I would still want to use tools from Big Sur or newer to verify or repair the APFS container which contained Big Sur. I think this would be good practice for macOS or any other operating system.

† Here I assume the write would be to a safe location. For example, while booted to Mojave, copying a file from a Mojave users desktop to a the same users Big Sur Desktop folder.

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  • Thank you for the hint, that it only affects system disks. Nevertheless this is a bug. An OS shouldn't changes a foreign disk without asking.
    – Adrian Zaugg
    Dec 30, 2021 at 11:31
  • Don't you know how to revert the changes the newer system makes to the disk?
    – Adrian Zaugg
    Dec 30, 2021 at 11:34
  • Your question if very confusing. Are you using two or more Macs? If so, the explicitly state which Mac the external drive is plugged into when booting macOS. Also, which version of macOS are you booting and where does this macOS reside. I understand you know what you are doing, but what you are doing is unclear to the readers.
    – David Anderson
    Dec 30, 2021 at 11:48
  • It does not matter if it's on the same mac or not, but if it is easier to imagine: Start a Mac with Mojave in target disk mode. Attach it to another Mac running Big Sur. Wait until you see the Mojave Mac's disk gets mounted. Eject the just mounted disk. Dettach. Switch off the Mac in target disk mode. Boot it and you see the message under Mojave.
    – Adrian Zaugg
    Dec 30, 2021 at 11:54
  • Did you actually do that or are you assuming what will happen?
    – David Anderson
    Dec 30, 2021 at 11:55
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I should have asked for clarification if the disks were data disks or system disks. This may need a major edit or deletion…

Not connecting older bootable drives to newer OS is a larger penalty than just not upgrading them…

Your question lacks some technical details but at the highest level, Big Sur implements a snapshot of the system to sign and attest that it’s unchanged at boot. The older OS do not have code to handle that situation and will break the drive for Big Sur if you somehow bypassed that restriction. This one way transformation does not happen on any disk except for ones where you run the OS installer.

The solution is to not install or upgrade Big Sur on any external drive that needs to be used with older OS and as you already know - erasing the drive or using a backup is the way to go back. Data drives can be shared in either direction, system drives only work forwards, not backwards.

Here is a nice article showing the technical details around this process that ends up creating a cryptographically signed snapshot of the system installed files that then gets mounted when needed to boot the OS in Big Sur or newer.

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  • The OP is not installing Big Sur on the drive. The OP has a drive with Mojave installed. This drive is mounted by Big Sur. The OP contends Big Sur modifies the drive so that when Mojave is booted, Mojave detects a change and issues the message. The message will occur even if the drive is booted on a machine that does not have Big Sur installed.
    – David Anderson
    Dec 30, 2021 at 12:25
  • Woah @DavidAnderson I missed that. I may need to delete this until I can retest since all my disks that threw this were system disks, I assumed it was the installer migration script at action. Thanks!
    – bmike ♦
    Dec 30, 2021 at 12:31
  • "system drives only work forwards" might benefit from a clarification? When using 10.6 it seems I could access up to 10.12 drives without much issues (10.7 needed for FV2 bits?). It appears to me that only for the moving target of 'APFS system volumes from 10.15+' this applies to (or APFS volumes for pre-APFS OSes)? Plus: previously it was easy to boot target-mode disks within macs with older versions on different computers.
    – LаngLаngС
    Apr 21 at 13:24
  • I’ll probably delete this @LаngLаngС I was answering data disks and David’s answer is great and explains this is only for bootable apfs
    – bmike ♦
    Apr 21 at 15:28

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