I've seen many people complain about this issue, and so I worked with Apple Care on this and they provided the fix below.
Power off iMac (hold down the power button until it shuts off)
Power back on
When you hear the chime, press Command+S. This will take you to boot the iMac into a command line.
Once at the command line, run the commands below.
Note: commands are case sensitive, so type them exactly as shown below. Also, you will need to hit Enter after each line.
mount -uw /
cd /Library/Pre... read more.