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?

  • what happens when you put back the 1600 MHz sticks ? They wont harm your iMac.
    – Ruskes
    Commented Jul 9, 2014 at 18:27
  • When I install the 1600 MHz RAM back into the computer, it boots just fine. I will try resetting the PRAM/NVRAM later when I get the chance and see if that allows for slower memory stick to be accepted.
    – G Hinson
    Commented Jul 9, 2014 at 18:34
  • Sounds like that 1333MHz RAM is bad. Having 1600MHz chips isn't a bad thing, the chip speed just drops down to the fastest that the machine can handle.
    – Fyrefly
    Commented Jul 9, 2014 at 19:07

1 Answer 1


I managed to get the slower RAM to work by resetting the PRAM/NVRAM: http://support.apple.com/kb/ht1379

Works like a charm now.

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