Mac Pro Memory upgrade

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I have a mac pro early 2008 and I am trying to add more memory. I currently have a pair of 2Gb on top and a pair of 1Gb on the bottom. If I add another pair of 2Gb will they need to be on the bottom parallel from the pair of 2Gbs I have on the top?
 
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I read similar txt before I started. I installed the additional 2Gb as all sites recommended in the placement sites recommended and it isn't showing up when I check for it after re-starting the machine. Before I turn it off and start over I was hoping someone could answer the question the diagrams from my various web searches have failed to answer.

The diagrams make it look like the order doesnt matter as long as the pair is grouped together. However my pair of 2Gbs aren't working when I place them next to the other pair of 2Gb i have on the top riser. My only guess/question would be that I need to put them on the bottom riser so they are parallel to the top, then move my pair of 1Gbs to the top riser.
 
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Silver M1 iMac 512/16/8/8 macOS 11.6
Macjones save your pennies and buy a pair of 4GB modules. Even better will be 8x1GB rather than mixed.
 
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2011 27" iMac, 1TB(partitioned) SSD, 20GB, OS X 10.11.6 El Capitan
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pm-r, it's not that it doesn't make sense its that the two additional 2Gb's aren't working when installed. The instructions have been followed, I am just trying to trouble shoot why it isn't working.

The original RAM of 2 2Gb's and 2 1Gb's have been working just fine in their slots.
 
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2011 27" iMac, 1TB(partitioned) SSD, 20GB, OS X 10.11.6 El Capitan
Well, maybe the rules for adding RAM chips is similar to the old SCSI rules:
Rule 1: All connections must be terminated
Rule 2: If it doesn't work, ignore Rule 1.

But on the serious side, have a check at the xlr8yourmac.com site, a good source of Mac fixit stuff and specifically this article:
Mac Pro FB-Dimm Pairing Guide/Tests (w/mixed 1GB, 2GB and 4GB pairs)

And if still not enough, this provides lots of possible solution hits:
https://www.google.ca/search?client...&oe=UTF-8&gfe_rd=cr&ei=uh-TVKHYMbjQ-AOJ_YCQDA

Good luck, but the xlr8yourmac.com article looks promising. ;)
 
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OWC have tested various memory set ups in your model Mac Pro. In the chart have a look at test No. 5 when 8 x 1GB modules are used. Nearly identical to using 8 x 4GB modules in test 16.


OtherWorldComputing.com.
 

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