Today I installed this security update and all went well
UNTIL the last minute of the install. At that point my iMac just refused to complete the install, telling me that there was less than a minute to go. After 30+ minutes of waiting for that less than a minute to arrive,
I tried to shut down my iMac, but it was non-responsive. Since I had to leave the house, I unplugged it from the power source and hoped it came back to life when I returned.
It did boot right up, but took over 10 minutes to finish whatever it was doing and allowed me to launch any software or do anything. However, I found that iTunes wouldn't work, and when I looked at the app icon in the applications folder, it was just a generic icon. I finally went to the app store and checked and it said that I needed to restart my iMac for the software to install. I restarted and left it alone, and after 15 minutes it was back to my log in screen and now appears to be working fine—and iTunes now functions.
I'm not sure what happened, but if you are running El Capitan and install this and your machine hangs up, I wish you the best.
