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I've just ordered the 27" iMac, which has user-upgradable RAM slots.

There are four slots, and the default configuration that ships is two 4GB modules, and two empty spaces.

Can I fill the other two modules with 8GB chips and bring the total to 24GB? Or, is it not possible for me to mix two 4GB modules with two 8GB modules.

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2 Answers 2

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Yes, you can and it's quite common practice.

One concern might be how the memory channels are laid out, as it's generally better to have symmetrical RAM configurations to balance dual-channel access. (so you'd want each channel to be 4+8)

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    And how would I find that out (regarding the layout of memory channels..)?
    – Matt H.
    Commented Dec 4, 2012 at 1:05
  • This page hasn't been updated yet, but it will likely be one pair (e.g. the original 2x4GB) in the top slots and the other pair (e.g. 2x8GB) in the bottom slots.
    – gabedwrds
    Commented Dec 4, 2012 at 1:30
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    Maybe a better answer: the stock RAM should come preinstalled correctly for dual channel mode, and you don't need to move it. Just add the new sticks in the open slots.
    – gabedwrds
    Commented Dec 4, 2012 at 1:40
  • Thanks. 24GB Should be WAY more than enough. It takes a deliberate effort to run low with 16GB in the MBP I have at work.
    – Matt H.
    Commented Dec 4, 2012 at 3:08
  • Hi! My plan is to buy the new 27" iMac with minimum RAM (2x4GB) and replace it completely with 4x8GB Corsair. Is it possible to replace ALL the slots? Or is it just possible to replace the remaining two ones? Thanks
    – fra
    Commented Dec 8, 2012 at 8:21
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This sure is possible and I have just installed a pair of 8 GB modules on my iMac a few minutes ago.

By not removing the existing two modules, the new cards just plug into the vacant slots - maintaining the dual channel configuration for a total RAM size of 24 GB.

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