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

    Joined:
    May 17, 2013
    #1
    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

    Joined:
    Nov 25, 2012
    #2
    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

    Joined:
    May 17, 2013
    #3
    Thanks. That's exactly what I was looking for. Can't believe I missed that detail throughout my searches.
     

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