The iMac User Guide (late 2009 models) indicates the two front slots (side-by-side, nearest the display) are to be filled first. Those are the slots your original RAM DIMMS should have been in. Presumably they are Bank 0. You can test this yourself by populating those positions, starting your Mac and checking System Information > Memory.
If you populate all 4 slots, technically it doesn't matter which DIMM is in which slot ... your Mac will work with whatever configuration you use.
Whether or not one configuration is "more optimal" than another is very hard to tell. Some years ago, with a couple of early iMac G5 models there was an optimal way to populate the slots; the Mac would work regardless but if the DIMMS were inserted in matched pairs memory speed was maximized. It was a consequence of the then-memory architecture of those iMacs. I have not heard of this being an issue in any Macs since that time.
In the absence of any definitive information to the contrary, I'd be inclined to insert the DIMMS ... in matched pairs ... with the 4GB DIMMS in Bank 0 ... and the 2GB DIMMS in Bank 1 ... "just in case" This is the same as what Crucial advised ... and ... they really know their stuff!
Thanks Martin, your reply was very helpful. You say, " If you populate all 4 slots, technically it doesn't matter which DIMM is in which slot ... your Mac will work with whatever configuration you use."
That is effectively what I have done, and yes the system does still work, as confirmed by the diagnostics. It is Crucial who are saying that the cofiguration, should be different and I obviously, want to optimise performance. However, I do not want to keep changing memory sticks.
You finish by saying insert the 4GB DIMM's in matched pairs, in BANK 0. As I am not a 'techie' can you confirm, that a BANK is 2 slots one above the other and that Bank 0 are the slots nearest to the On Button, i.e facing the screen, the left two slots? When you install the memory and put the screen face down, this reverses the situation and can be confusing, so clarification, would be a tremendous help.
The banks are the horizontal pairs of slots, side by side, parallel with the face of the display.
According to the manual I linked in my earlier reply, when you place your iMac (i5, 27", late 2009) face down, the two slots next to each other, closest to the face of the display, are Bank 0. You would be inserting the two DIMMS next to each other (short sides next to one another) as indicated in the picture below (see p 40 of the manual).
Bank 1 is the similar pair of horizontal slots closest to the back of your iMac.
Thanks for the info, but I am now totally confused! I thought that Banks were vertical not horizontal. My configuration, based on your latest reply, is:-
Bank 0 DIMM0 4GB
Bank 0 DIMM1 2GB
Bank 1 DIMM0 2GB
Bank1 DIMM1 4GB
I definitely thought that I had put the 2 x 4GB memory sticks in the top two slots, which you now say is Bank 1, but my diagnostics do not show that.
Trying to look at it simplistically, Crucial are saying that I should have 2 x 4GB in the 'bottom' slots on your diagram i.e. Bank 0 and the 2 x 2GB's in the top 2 slots i.e. Bank 1.
Another point I would like to ask is, when inserting the memory sticks, using your diagram, would I be better, putting in the top bank first, or the bottom? I do find it difficult to get them to click in very easily and wondered if there was a technique, which would make it easier.
Thanks for your very considerable help. It is very much appreciated.
An update. I have just spoken to my 3rd person at Crucial, who having checked with my model, basically confirms your view that, 'If you populate all 4 slots, technically it doesn't matter which DIMM is in which slot ... your Mac will work with whatever configuration you use.'
It appears to me that Crucial have not realised that my model is a Quad core. Anyway the latest from Crucial is that my configuration is OK. I apologise for the confusion, but thank you for your patience and I now feel that I better understand my computer.
I have the same 12GB RAM in my i5 iMac. With the iMac face down, the original modules are closest to the screen horizontally inserted and the upgrade Crucial modules are nearest the back of the iMac.
I had issues ( random shutdowns ) when they were reversed having the new bigger modules closest to the screen and was advised in another thread to keep the original Apple ( read Samsung ) modules in the two horizontals closest to the screen.
So far working fine.
Martin and Airsculpture,
Thanks for your advice and experience. Interesting that my memory sticks are in totally the opposite slots to yours. However, I shall continue to keep mine where they are, as they seem to be performing OK and the systems diagnostics confirm that. Much appreciate you taking the time to respond.