Tuesday, December 09, 2008

Mac Pro 8-core RAM not recognized?

WARNING: Do not attempt this if you are not comfortable with hardware hacking. I will not accept any responsibility for anything you do trying to follow this article.

I went to the store yesterday to get more RAM for my Mac Pro, an essential need really if you are editing video as I am for Duane's World TV. I discovered that installing the RAM into an 8-core Mac Pro is not only somewhat difficult, but poorly documented. This step by step guide is aimed at helping save others the 4 hours it took me to get an answer from Apple.

First, the basics. For the new Mac Towers, RAM is distributed into two Printed Circuit ("Riser") cards, each with 4 DIMM slots that you can put RAM into. Before you begin, there are a few basics you should understand. Ground yourself!!! I cannot emphasize this enough. There are commercial products available or you can roll your own. I decided to use a 1/4 inch guitar cable taped to my wrist (metal touching skin) and grounded by connecting it to an Ampeg Bass head ground connection. You can also use a metal plumbing pipe but be careful that it is metal all the way into the ground (sometimes they use PVC in the middle). Here is my MacGyver setup:

This keeps static and other electrical flows from ruining the RAM. If you want to know why, read this article here. Believe me, this is not to be taken lightly as I have personally ruined RAM chipsets.

When you remove the panel to the Mac, you will see two RAM banks, an upper and a lower. Note that there are white dots that are aligned with the surface of the chassis to let you know that the boards are installed properly. See below:

On the newer machines, you can simply pull them out by placing your fingers in the holes as shown below and pulling gently. Be sure to read service bulletins to understand if a clasp or other lock in device gets added after this article runs to avoid breaking these board. In general, you must treat these PC ("Riser") boards very gently. Do not carry them across carpets then let them touch a metal window frame or bend them.

Gently remove both the upper and lower boards and place them on an insulated surface. Now here is the weird part. I always knew RAM had to be installed in matched pairs and I had a 2GB in DIMM 1 on both left and right and a 1 GB on DIMM 3 on both left and right so I installed an extra 1 GB on each side in DIMM 4 on both left and right as shown below.

This should have been 8 GB of RAM in total
((DIMM1-Left = 2GB + DIMM1-Right = 2GB) + (DIMM3-Left = 1GB + DIMM 3-Right = 1 GB) + (DIMM4=Left = 1 GB + DIMM4-Right = 1 GB) = 8 GB. WRONG!! It was not recognized. When I started up my system, this is what showed up.

It showed that DIMM 2 and 4 on both sides were empty and I only had 6 GB of total RAM. I tried various configurations before giving in and calling the shop that sold me the RAM. The guy there knew nothing about installing RAM and just told me I probably had bad RAM. I tried to explain that each RAM stick worked in positions 1 and 3 on both sides as long as it was balanced however he failed to grasp the most primitive forms of logic. I then called Apple who were much more knowledgeable. It turns out that not only do Left DIMM 1 and Right DIMM 1 have to be matched on 8-core systems, but DIMM 1 must match DIMM 2 on both right and left. This left me going back to the store to buy two more 2 GB RAM strips and installing them as shown below:

Place them back VERY gently into the two slots. Don't force these into the chassis. Before the final push (you will have to push hard) the white dots should be about 3 mm away from the metal chassis edge. If you try to do the hard push when they are 15mm away, you can break something. Also - before you try to place them back into your machine, make sure each RAM strip is locked into place as can be evidenced by the clips lining up as shown in the second photo below. If the clip is left open as shown in the first photo and you try to force it, you can easily break something that will cost a lot of money to fix.

After all is safely back into place, start up your Mac. Here is what I was rewarded with!

12 GB of RAM with 8 cores!! I am now faster than you!!! Bwahahahaah!!!


  1. I always buy my Mac RAM from OWC (http://eshop.macsales.com). They've been serving the Mac community for as long as I can remember, have a knowledgeable staff, and offer DYI videos on doing nearly everything.

    Example- installing memory and hard drive in a Mac Pro:


  2. Alternative title for this blog post:

    How I convinced my Mrs that I just HAD to upgrade to 12GB of RAM on my Mac Pro :)

  3. @JohnO

    Damn - wished I had found that link earlier. Would have saved me a ton of time. Thanks for posting.

  4. If you had moved your new 2GB modules down to slot 3 you would have been fine with your initial purchase. Usual instructions for installing RAM are to fill the slots from the bottom up.

    BUT... you are right that the slots seem now not to be paired... but grouped in 4's.

    I bought 2 x 4GB DIMM's and had to install them in slots 3 - 1 on each of the pull out cards.

    1 - 1GB
    2 - Empty
    3 - 4GB
    4 - Empty

    On each card... for those that don't want an excuse to go and buy more RAM... but just want it to work.


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