But that is the curve ball!
Really, both sets of RAM are showing fine, but ONLY when installed seperatly.
I will probably send an e-mail to cosair and apple, just to see if they know anything about this.
I am really suprised no one else upgrading their new iMacs is having this problem. Or maybe the just haven't noticed yet
Well, I've not heard anything about Corsair, although the two most recommended RAM vendors here are OWC (macsales.com) and Crucial. So, if the RAM shows ok separately, then I'd start blaming it on the Corsair not being totally compatible - did you buy it someplace where you can return it? I'd try different RAM. Macs are terribly finicky about RAM.
Then, although the problems were different, there were RAM related problems in the late 2009/2010 iMacs (that's why I have 12 GB rather than the max 16) - I'm linking to one of many threads here from around that time. There aren't enough data yet on the 2012 iMacs since they are just now shipping, but although your problem is different, it is just as strange as that last one.
Agreed. You should always install ram in the top two slots first (the two slots closest to the screen). That is how apple ships them, and in some cases, the mac will not start without both slots occupied. As for the speed issue, what happens if you put the Cosair sticks in the primary two slots (the two horizontal slots closest to the screen) and the OEM in the secondary slots (the slots closest to the back of the unit)? Maybe not having the OEM ram as the primary ram would fix it.
Just to let you know you are not alone in seeing this. In this thread Best way to configure RAM? the user was seeing the same speed drop listed in the display. In his case he was using an odd amount of memory (3 sticks) and we thought that might have something to do with it.
But as you're now seeing this with an even amount it appears something is up with the new iMacs.
Sounds like some anomolies.
Fortunately a more positive result but I just got a late 2012 27" 3.4GHz i7 imac with base spec of RAM being 8Gb (2x4GB). Says mine was assembled in China if that makes a diff?
Ram slots on back of machine in out-of-box configuration was (and after testing pulling modules in and out);
top slot -------- empty Bank 0 / DIMM1
2nd 4Gb Bank 0 / DIMM0
3rd -------- empty Bank 1 / DIMM 1
bottom 4Gb Bank 1 / DIMM 0
in apple/about this mac/memory this displays as arranged horizontally like;
and reads "Your Mac contains 4 memory slots, each of which accepts
a 1600 MHz DDR3 memory module". 1600MHz also showed up in Utilities "System Information/Memory"
I too bought a dual channel Corsair 16Gb Notebook memory 1600MHz SODIMM DDR3 (2x8Gb) CMSA16GX3M2A 1600C11. I put them in the empty slots and no problems, all read as 1600MHz, 24Gb total. I tried all different configurations of the modules (even from top 8Gb/4Gb/empty/empty) but always showed up as the correct total ram and all at 1600MHz.
I also read in another apple forum somewhere that both Crucial tech and Apple tech had said that out-of-the-box apple modules are in the Dimm slots that are accessed first and both were recommending to move them if you were adding higher ram modules so i guess the best performance would be in the configuration;
top slot 4Gb Apple Bank 0 / DIMM1
2nd 8Gb new Bank 0 / DIMM0
3rd 4Gb Apple Bank 1 / DIMM 1
bottom 8Gb new Bank 1 / DIMM 0
I am going to memorize part of your post because I had not seen that - with that I mean the fact that you said the OEM populated slots were 2nd and 4th, but showed as matching horizontally in About this Mac. I wish Apple would match their "pictogram" to the actual visual configuration and refer to it as such, as in: Slots 1 + 3 and 2 + 4 should match.
As to the reason I posted the link to a 2 year old thread: it seemed there were anomalies there as well and some mfr vs. OEM and/or size configurations would work and others wouldn't. At the time, those were the "just introduced" iMacs as well.
I see that Apple now finally has published instructions on how to install RAM in the new 27"; however, they no longer include graphics and there is no mention of which slots are occupied by the OEM RAM.