32

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?

Improve this question
1
  • 2
    What are you trying to achieve by mounting root?
    – benwiggy
    Aug 8, 2021 at 8:15

2 Answers 2

Reset to default
26

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
Improve this answer
10
  • 7
    WARNING If you follow these instructions you will not be able to reenable FileVault!
    – user3439894
    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
    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
    Feb 12, 2021 at 2:12
  • 1
    @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
    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
    Apr 10, 2022 at 15:22
19

My company requires 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.

Improve this answer
5
  • 5
    I don’t know why this was downvoted. Maybe because the answer is formulated in an anecdotal way, but AFAICS it points to the preferable way of creating symlinks or directories under / on Big Sur, namely “synthetic firmlinks”, see man synthetic.conf or derflounder.wordpress.com/2020/01/18/…
    – hans_meine
    Apr 3, 2021 at 18:55
  • 5
    This is the actual correct answer that solves the XY problem here
    – Andrew Faulkner
    Jun 9, 2021 at 18:24
  • This is a solution to the problem, but not an answer to the question. So I understand it not being the top answer. Thank you though, as this solves my exact problem in a less hacky way.
    – tschundler
    Jul 26, 2021 at 23:16
  • I'm not sure this helps you exctaly as I don't know the reason you want writeable, but if you just one folder like me that needs rw for projects, just open "System Preferences" -> "Disk Utility" and create a volume on your disk. Very easy and painless.
    – Marco
    Feb 5, 2022 at 19:53
  • Beware disinformation from posters on discussions.apple.com - see archive.ph/V5BWp for a bunch of misinformation on this topic - which Apple then censored entirely.
    – WHO'sNoToOldRx4Covid-CENSORED
    May 22 at 19:36

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .