Thank you. I was faced with a trivial issue of not being able to place custom icons for MIDI Devices in /System/Library/... I noticed somewhat of a mirror of the path in /Library/... Yet I still started to slide down the rabbit hole of trying to add to /System/Library/... The key was the restart, which your post led me to try before I went too far trying to gain privileges to /System/Library/. Thanks again

Thanks, worked for me.