MacOS Catalina read-write mount could not be mounted in single user or recovery mode with permission denied

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I need to have writable access to the file system in recovery mode, but I always get the error

mount_apfs: volume could not be mounted: Permission denied.

I am aware of others who solved it like this: Read-only file system" with SIP disabled in macOS Catalina

i.e.:

  • start in recovery mode (Cmd-R at startup)
  • open terminal and disable SIP with csrutil disable
  • reboot into single user mode (Cmd-S at startup)
  • check SIP is disabled with csrutil status
  • try to mount the volumes with read/write:
sudo mount -uw /
sudo mount -uw /System/Volumes/Data

Neither of the two volumes can be mounted, I always get permission denied even with SIP disabled.

Interestingly, even if I boot the Mac from an external SSD (in this case an old Sierra installation) I cannot mount the disk as read/write.

Any further suggestions?

macos-catalina macos
1 Answers

The problem in this case was a defect SSD, which switched into readonly mode after only 36 TB written, despite having a design of 1200 TBW.

Unfortunately, MacOS did not report this. When looking at System information > Storage > my SSD > SMART status the system still showed "Verified", which is supposed to mean that everything is ok.

It was not.

I determined this by installing smartmontools and running a check:

brew install smartmontools

smartctl -a disk1

it showed e.g.:

SMART overall-health self-assessment test result: FAILED
- Available spare has fallen below threshold
- media has been placed in read-only mode

- Data Units Written: ... 35,4 TB
- Available Spare: 1%
- Available Spare Threshhold: 10%

So issues

  1. bad luck with SSD
  2. bad error reporting by MacOS
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