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This question already has an answer here:

I'm experienced with Windows, not so much with Mac. I've just been given a 27" iMac (it's a mid 2011 model) by someone who no longer needs it. It has 4 memory slots and presently two are occupied with 2Gb modules each (4Gb total RAM).

Another friend has offered me a 4Gb module and a 2Gb module. I've checked the specs and they are compatible with this model iMac.

Is it an option to put each of these in the 2 remaining slots, so that 3 slots have 2GB and 1 slot has 4Gb?

While this question: Is using mid 2012 MacBook Pro RAM in mid 2011 iMac 27" an option? is similar, it does not address the following question I have:

And if one of these is bad memory, can I only use one? By this I mean 2 slots have the original 2Gb modules, one slot has either the 2 or 4Gb module, and the last slot is empty.

marked as duplicate by Allan, Jaime Santa Cruz, bjbk, bmike Jun 5 '17 at 21:21

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • For MacOS, yes. I don't know how Windows handles mismatched RAM so I will let someone else finish this answer – NoahL Jun 5 '17 at 5:45
  • Mactracker lists a Mid-2011 iMac 27", not Late-2011, which is a 21" screen. Both models have four slots for 204-pin PC3-10600 (1333 MHz) DDR3 SO-DIMM. The 27" model maxes out at 16GB (Apple spec) while the 21" maxes out at 8GB. Both models can actually use double Apple's max spec (32GB and 16GB respectively.) – IconDaemon Jun 5 '17 at 12:56
  • If it helps (I asked the 'duplicate' question), in my case I added the mid-2012 MBP RAM into the mid-2011 27" iMac and it works just fine, but I don't have an empty slot remaining. – user205310 Jun 6 '17 at 12:42
  • There is no requirement, whatsoever, to install memory in pairs Meaning, you can have open slots. – Allan Jun 6 '17 at 14:37
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There are multiple iMac models, but in general, look in the apple menu and select 'About this Mac', then (in the window that opens) click on the 'Memory' selection. This will show your memory arrangement and offer "Memory upgrade instructions" if the computer is internet connected.

If the memory arrangement recognizes all the memory you have installed, it's working (though matched pairs are often preferred).

  • My question was closed and I do not know if it will be allowed again after my edit. Do you know if it will work fine if I have 4 slots with 2Gb, 2Gb, 4Gb, and nothing? – user725960 Jun 6 '17 at 2:07
  • It depends on model; many models will work faster if identical pairs are used, but will recognize and use unpaired memory. If you install and do the steps above, it'll show you what memory is recognized and working. – Whit3rd Jun 6 '17 at 2:55

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