Can't write to /usr/lib I upgraded to OS X El Capitan and tried to install a new app (MPLAB X from Microchip) but I got an error because it couldn't copy a library to /usr/lib even when it asked for root password. I tried to manually copying the library using sudo cp libSEGGERAccessLink.dylib /usr/lib but it says cp: /usr/lib/libSEGGERAccessLink.dylib: Operation not permitted.

Is this a permission issue? How can I write to /usr/lib?  ls -ld /usr/lib returns: 

drwxr-xr-x  263 root  wheel  8942 Oct  2 15:49 /usr/lib
It is a new feature of OS X 10.11 (El Capitan): SIP.


  System Integrity Protection (SIP,1 sometimes referred to as rootless2) is a security feature of OS X El Capitan, the operating system by Apple Inc.


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  Among the protected locations are: /System, /bin, /sbin and /usr (but not /usr/local).


System Integrity Protection on Wikipedia

It can be disabled:
Stack Exchange thread

In summary, from the Recovery partition, open Terminal.app and run: csrutil disable

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Update: SIP has been expanded in High Sierra (macOS 10.13)

KB on Apple's Support Site