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After updating to big sur, I can no longer mount the root as writable (even with SIP disabled):

sudo mount -uw /
mount_apfs: volume could not be mounted: Permission denied
mount: / failed with 66

How can I fix this?

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2 Answers 2

Reset to default
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DISCLAIMER: have a good backup and plan to erase and reinstall your Mac from the internet before trying this procedure. A mistake can prevent your system from starting without assistance.

  1. Disable FileVault
  2. Reboot into recovery mode and run: 
    csrutil authenticated-root disable
  3. Reboot back into MacOS
  4. Find your root mount's device - run mount and chop off the last s, e.g. if your root is /dev/disk1s2s3, you'll mount /dev/disk1s2
  5. Create a new directory, for example ~/mount 
    mkdir -p -m777 ~/mount
  6. Run sudo mount -o nobrowse -t apfs DISK_PATH MOUNT_PATH, using the values from above 
    sudo mount -o nobrowse -t apfs /dev/disk1s2 ~/mount
  7. Modify the files under the mounted directory
  8. Run sudo bless --folder MOUNT_PATH/System/Library/CoreServices --bootefi --create-snapshot
  9. Reboot your system, and the changes will take place
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  • 7
    WARNING If you follow these instructions you will not be able to reenable FileVault!
    – user3439894
    Commented Dec 23, 2020 at 12:55
  • 2
    How is this managed to work? You cannot unmount something that runs your terminal and zsh/ bash /sh whatever you need to run it? I cannot manage to get this working as mount_apfs: volume could not be mounted: Resource busy failed with 75
    – Dominik Bucher
    Commented Feb 5, 2021 at 1:05
  • 3
    I'm getting exactly what @DominikBucher says on an M1 (Apple Silicon) with latest Big Sur installed. When mounting from recovery and modifying, for instance, files under /System/Library/LaunchDaemon/ with both SIP and SSV disabled (SSV = eclecticlight.co/2020/06/25/…), any modifications performed are not persisted once I boot back into OSX :/
    – brainstorm
    Commented Feb 12, 2021 at 2:12
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    @brainstorm Like you, I was able to successfully mount r/w in recovery and modify files, but lost all the modifications when I rebooted. I think it is because step 8 (the bless command) fails to even create a snapshot, much less bless it. I also tried another way to create a snapshot, /System/Library/Filesystems/apfs.fs/Contents/Resources/apfs_systemsnapshot -v <volume mount point> -r "" from tonymacx86.com/threads/… but that also gave an error, so I rebooted back without a snapshot of my changes, and lost them all ..
    – auspicious99
    Commented Apr 28, 2021 at 16:05
  • 1
    I got it to work finally. The problem was that authenticated-root was not properly disabled. I run MacOS in VMWare, and disabled SIP using this guide: itectec.com/askdifferent/… but disabling authenticated-root was missing from the hex value stored to NVRAM. Using value ff0f0000 instead of 7f000000 did the trick.
    – GolDDranks
    Commented Apr 10, 2022 at 15:22
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The company I work for requires us to read/write environment informations under /data/webapps/appenv. Therefore, this workaround shared by a colleague of mine might not answer this question directly, but it is super useful if you want to write something under your root path!

  1. Create a file: sudo vim /etc/synthetic.conf.

  2. Write content: <dir>⇥/System/Volumes/Data/<dir>. Notice: the ⇥ is a tab!

    In my case, it looks like this: data⇥/System/Volumes/Data/data

  3. Go to /System/Volumes/Data and do sudo mkdir <dir>.

  4. Then do sudo chmod -R 777 <dir>.

  5. Finally, reboot, and your directory <dir> should now be accessible.

PS: See see man synthetic.conf or https://derflounder.wordpress.com/2020/01/18/creating-root-level-directories-and-symbolic-links-on-macos-catalina/ for details about synthetic.conf.

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