I'm on Big Sur 11.2.2, Single User Mode. When I use /sbin/mount -uw /, it says mount_apfs: volume could not be mounted: Permission denied and failed with 66.

I have read this post. But mount -uw /System/Volumes/Data gives mount: unknown special file or file system /System/Volumes/Data/. Also, csrutil enable --without fs in recovery mode changes nothing. Errors still occurs. (Previous tries are based on SIP entirely disabled.)

The commands mount -P 1; /usr/libexec/init_data_protection; mount -P 2 in the prompt during startup works. However, I don't know what it's mounting.

PS: current SIP status:

    Apple Internal: disabled
    Kext Signing: enabled
    Filesystem Protections: disabled
    Debugging Restrictions: enabled
    DTrace Restrictions: enabled
    NVRAM Protections: enabled
    BaseSystem Verification: enabled

This is an unsupported configuration, likely to break in the future and leave your machine in an unknown state.
  • Does your machine have a T2 chip?
    – twlscnds
    Commented Mar 24, 2021 at 10:41
  • To add to @TwlvSeconds's comments, Single User Mode (SUM) doesn't work anymore on Macs with T2 chips, as it can't bypass the secure boot of T2 chips. The closest you can get is to perform your functions in recovery. Also I had though SUM has been depreciated since pre-Catalina or am I wrong on this?
    – AVelj
    Commented Mar 24, 2021 at 12:04
  • Your root volume is a snapshot of the system volume, which is not writable by design. Basically, you have to mount the system volume, make changes, create a new snapshot, and boot with that new snapshot for the changes to take place.
    – Joy Jin
    Commented Mar 25, 2021 at 6:18
  • @TwlvSeconds No. (MacBook Pro (15-inch, 2016))
    – xiaoyu2006
    Commented Mar 25, 2021 at 11:10

1 Answer 1


The limitations of Single-User Mode and the changes in Mojave & Catalina have been covered before. In particular why you can't unmount the system volume. I don't think it needs repeating again (unless someone want to add changes in Big Sur). Read this:

fsck_apfs not working from terminal in single user mode

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