I have a mid-2011 27" iMac with 3.4Ghz Core i7 and 8GB memory as configured in the originally purchased machine. I want to upgrade to 16GB and have found a number of threads about OWC vs Crucial. That helps. My problem is: I already have 8GB originally installed by Apple at the time of purchase of the machine on 2 of the 4 memory slots. Will there be any compatibility problem with installing an additional 8GB from OWC in the additional 2 memory slots? Or, do I have to buy the full 16GB from OWC or Crucial and throw the Apple memory out? Thanks for your help.