Q: Anyone else had an issue installing an 8GB RAM module in a 6-core Westmere Mac Pro?

Remove all the others and try installing it by itself in the frontmost slot.

Use a bright light to be certain that it is completely seated and the extractor clips "pop up" into the full upright postion.

As I recall my 8G modules had labels on both sides, so I was trying to install one of them backward.  Look for the gap in the bottom to align properly.

Do you have any other RAM installed?  The 8 GB can't be mixed with other types of RAM.

<http://support.apple.com/kb/HT4584>

Actually: don't mix UDIMMs and RDIMMs.  You can mix 2's 4's and 8's.

"May" but have seen somebrands do not allow mixing even now, so trh with only a single 8GB DIMM which is helpful test anyway and eliminateunknowns and insure it works o  its own

chariii wrote:

 

So the 8GB module worked on it's own. Adding any of the 2GB or 4GB chips resulted in the blinking status light. Guessing I need to send the 8GB back and get 2 4GB chips, or order an additional 8GB to get to the desired 16GB. Thanks for the help everyone. Wishing Apple store noted the conflict to begin with.

I don't know if you can get a refund rather than replacement with different sizes for your Apple DIMM, but you should check out http://eshop.macsales.com/shop/memory/Mac-Pro-Memory#1333-memory for quality, prices, and real customer support of a lifetime warranty.

Their 8GB DIMMs are what I'm using in my 6 core Mac Pro.

I have 2's and 8's working together in my 12 core Mac Pro.  The 2's are originals from Apple, the 8's are from OWC.

Gerald Edgar wrote:

 

I have 2's and 8's working together in my 12 core Mac Pro...

Interesting. Since you've got two processors while chariii and I only have one, are the 2's grouped together in one memory slot group and the 8's in the other? Apple's RAM installation instructions say "Each processor’s memory controller has three memory channels" so if the same size DIMMs are grouped together with their own memory controller, any conflict caused by mixing them in the same controller is avoided. OWC no longer warns against mixing sizes with their 8GB DIMMs as they do with their 16GB modules but they did when I ordered mine.

I have six 2's from Apple, and added two 8's from OWC.  The computer came on and said they were not in the best locations for maximum effectiveness, and showed where they should be.  So I now have the 8's in sockets 2 and 6, and the 2's are in the other six sockets.  Don't know how these correspond to "three memory channels"?

According to the online invoice from OWC, these are: 

16GB (2x8GB) DDR3 ECC PC10600 1333MHz SDRAM ECC for Mac Pro 'Nehalem' & 'Westmere' models 

I cannot tell you more without opening up the case...

Gerald Edgar wrote:

 

...Don't know how these correspond to "three memory channels"?

It  shows that even when one of the three memory channels that each processor's controller handles is shared, an 8 GB DIMM can be mixed with other sizes without ill effect. The three channels are slots 1, 2, 3 for one processor and 5, 6, and 7 for the other. Slot 4 shares a channel with slot 3 and slot 8 shares one with slot 7.

I'd assume that the original configuration from Apple was slots 1, 2, 3, 5, 6, 7. Adding the 8 GB DIMMs in the still empty slots would put them in 4 and 8, meaning an 8 GB DIMM was sharing a channel with a 2 GB DIMM. That arrangement still worked but was not ideal. Perhaps Apple has tweaked the memory system for its newest Mac Pro's (I'm assuming yours is a 2012) or OWC has done the same with its DIMMs, or both.