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Did you make sure to install the RAM in slot 1 and 3, or 2 and 4? They should be staggered for correct pairing.

Yes
Slot 2and 4

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Run Rember which is a memory tester for Mac OS X....it will tell you if you have a bad stick. I had one in the Corsair Vengeance Kit for my Mac Mini Server...replaced it and all was well.

I just done full apple hardware test
A memory error was found.
Not sure it's because bad ram or compatible issue
 
Yes
Slot 2and 4

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I just done full apple hardware test
A memory error was found.
Not sure it's because bad ram or compatible issue

Are you getting a memory error with all 4 slots being used? Or just the Corsair by itself in slots 2 and 4?

If it's just the Corsair by itself, try one stick by itself, and run the Apple Hardware Test.

The more info you can gather, the easier it will be to figure out what's going on here!

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+1 for crucial, flawless performance and excellent customer services.
I always order directly from crucial.com

PS does the new late 2012, stock iMacs come with x2 8 GBs ram chips, thus allowing us to add a further two?

I've had bad Crucial and bad Corsair sticks in the past. No company is perfect.

Late 2012 27" iMacs come with 2x4GB standard, or upgrade to 2x8GB, or upgrade to 4x8GB.
 
So I just picked up my late-2012 iMac from the Apple Store. They have evaluated it for the past 24 hours (right at) as it was continually freezing. They ran it for several hours with the Apple Ram (8GB) and never had a problem. They ran it with the Corsair CMSX16GX3M2A1600C10 (32GB) for several hours and it froze several times. They also ran it with KomputerBay Ram (32GB) I bought off of Ebay and it froze several times. The Apple Genius indicated that is more than likely the RAM since everything else checked out fine. He told me that I was one of only two late-2012 iMacs that had come in the store and the other work order was for the same issue. He said that his early observation is that these new iMacs will be very fickle when it comes to Ram. The Ram that froze all ran at 1.5V. He stated that while that shouldn't have caused a huge issue, he would only purchase Ram at the 1.35V requirement. I will be ordering 32GB of the Crucial Ram for my late-2012 iMac now.

This post is in no way meant to diminish or tarnish another brand, but simply my experience. I have used Corsair and KomputerBay in the past with no issues.
 
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