System Integrity Protection (SIP)
Apple has implemented a new security feature in 10.11, System Integrity Protection, which prevents even super users from altering the contents of files in key System folders. The flags for restricting the folders is applied at the time of installation and cannot be overridden. SIP is designed to protect System files from malware and viruses.
Disabling SIP requires booting into the recovery partition to run the Terminal command below.
# csrutil disable
which should confirm with status message:
Successfully disabled System Integrity Protection. Please restart the machine for the changes to take effect.
Reboot back into the El Capitan system partition.
To re-enable SIP, boot back into the recovery partition and set state to enabled:
# csrutil enable