2008 Mac Pro - Why do 2GB sticks of server FB-DIMM RAM work but 4GB sticks do not?

Discussion in 'Mac Pro' started by -PM-, May 17, 2013.

  1. macrumors newbie

    May 17, 2013
    Hey guys, new to the board, but long time follower. I have a question that has been bothering me for almost a year.
    I have been able to install 2GB sticks (up to 8 for a total of 16GB) of PC2-6400F from random servers (HP, IBM, etc.) in a 2008 Mac Pro without a single issue.
    But when I try to use 4GB sticks (also from random HP or IBM servers) of PC2-6400F, they are never recognized. All 4 of the red lights are on for each of the modules and the power LED blinks. No startup at all.
    I have tried every memory configuration possible with no results. I have tried over a dozen different working pairs of matching 4GB sticks on 4 different 2008 Mac Pro's without any success.
    Can anyone shed some light on this matter? I have searched and searched, but couldn't find anyone that has an issue like mine.
  2. macrumors 6502a

    Nov 25, 2012
    The answer to your question is right in this post, any quad rank (4R) FB-DIMM modules won't work with the MP, but dual rank (2R) will. You should read the whole thread to see your options.
  3. thread starter macrumors newbie

    May 17, 2013
    Thanks. That's exactly what I was looking for. Can't believe I missed that detail throughout my searches.

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