What is the difference between SIP and SSV?

SIP (System Integrity Protection) restricts root access to system files. SSV (Signed System Volume) adds cryptographic verification that the system volume has not been modified.

Where should I put custom files?

Use /usr/local, /Library, or your home directory. For system-level mounts, use /etc/synthetic.conf to create firmlinks.

Mac Error 69877 — APFS Operation Not Permitted on Signed Volume

Errordisk

Overview

Mac error 69877 occurs when trying to modify a Signed System Volume (SSV) in macOS Big Sur or later, as the system volume is now cryptographically verified.

Key Details

Starting with macOS Big Sur, the system volume is a Signed System Volume (SSV)
SSV is cryptographically verified at boot — any modification breaks the seal
Error 69877 appears when trying to mount the system volume as writable
SIP (System Integrity Protection) must also be disabled to modify system files
Even with SIP disabled, SSV prevents direct system volume modification

Common Causes

Attempting to modify files on the sealed system volume
Legacy tools or scripts trying to write to /System or /usr (excluding /usr/local)
Trying to mount the system volume as read-write
Older administration tools not compatible with SSV
Manual partition operations on the APFS system volume group

Steps

1Understand that modifying the system volume is intentionally blocked for security
2Use the data volume (/System/Volumes/Data) for custom files instead
3If absolutely necessary: disable SIP (csrutil disable in Recovery) then disable SSV (csrutil authenticated-root disable)
4After modifying system volume, re-seal it: sudo bless --folder /System/Library/CoreServices --bootefi --create-snapshot
5Prefer synthetic firmlinks (/etc/synthetic.conf) for custom mount points instead of modifying system