I recently upgraded an iMac to 16 GB of RAM using 1600 MHz clock speed sticks. It worked fine, but I came to find I was supposed to install 1333 MHz sticks instead. So I tried switching the current modules with the appropriate 1333 MHz memory modules and the computer won't boot up; the iMac would just beep until I unplugged the power.
I've tried various combinations with the 1333 MHz RAM and no luck getting it to boot. Is there a system preference that I have to reset in order for the computer to accept the RAM?