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.