Maximum memory config for Mac Pro 5,1 (mid 2010)? I have a MacPro5,1 with 8 cores (2 quad-core processors) running OS X 10.10.2.  From what I have read as per EveryMac.com's information about this unit it should be possible to have far beyond the normal maximum noted by Apple:


  Actual Maximum RAM
  
  ...
  
  Systems with dual processors, which EveryMac.com notes as the "Eight
  Core" and "Twelve Core" models, likewise officially support 32 GB of
  RAM, but again, OWC found they actually can support more. OWC first
  confirmed an actual maximum of 64 GB of RAM running Mac OS X 10.5
  "Leopard" and later increased this maximum to 96 GB of RAM running Mac
  OS X 10.6 "Snow Leopard" or higher. More recently, OWC yet again
  discovered these dual processor models can support up to 128 GB, but
  only when running a 64-bit version of Windows XP or later or Linux.
  Mac OS X only can support 96 GB of RAM.


I initially had an arrangement of 8 4GB DIMMs. These brought my system up to 32GB.

I then purchased 2 16GB chips in hopes to raise the amount of RAM, however my system would simply not start up at all.  Gave a fast power LED blink.  I subsequently returned this RAM and acquired 4 8GB DIMMs.  

Upon installing these, my system still shows 32GB of RAM, but the 7 &amp; 8 DIMM slots are showing empty now at this point.  Two of the DIMMS are being recognized while the others are not, even though there are 4 GB DIMMs in place and 4 4GB DIMMS:



What kind of issue is going on here?  Obviously we're getting mixed messages with the info above about being able to accept much larger memory configurations, but in practice things get quite whacky.
That machine will take 16's - my 3,1 does, to an unofficial maximum of 64GB [or more, according to Everymac, though I've not tested that myself].

The arrangement is not what you would initially think logical.


If you install equal-size DIMMs (for example, all 1 or 2 GB DIMMs) in your Mac Pro, Apple recommends that you fill the slots in the order listed in the table.


Note: If you install different-size DIMMs in your Mac Pro, follow the order in the table. If the DIMM configuration you install doesn't provide optimized performance, the Memory Slot Utility appears onscreen and recommends an improved configuration.



Source: Mac Pro (Mid 2012 and earlier): How to remove or install memory … &amp; previous similar confusion when I upgraded mine ;)