Upgrading RAM to a 6.1 MacPro

Macs are very fussy as to which RAM modules will work in them. many people here, including me, will only recommend using RAM from OWC or Crucial.com. they both supply RAM that is known to work. using RAM from other sources will be a craps shoot as to wether or not it will work in a Mac. even if the specs appear to line up.

The main issue on these desktop Macs is that DIMMs 8GB and smaller do not play nice with DIMMs 16 GB and larger. Larger DIMMs have so many chips on them that the chip outputs alone can not just be wired together, and an output Register is required. They are called Registered DIMMs.

so when your new ones arrive, remove ALL the old DIMMs and set them aside (or sell them back to OWC). Try the new ones a few at time to build confidence they are working before just installing all at once.

DIMMs in this model can be installed by the each. if you install two in adjacent slots that are close enough in specs, they will interleave and operate a few percent faster. The benefit of having enough RAM for the work you do completely swamps the small percentage you get from this pairing.

Users report difficulty seating the DIMMs completely. Check that the alignment slots are aligned, then Push Hard until fully seated.

if the nominal specs are the SAME as the ones for sale at OWC, they are fairly likely work, BECAUSE:

Desktop Macs like the Mac Pro typically contain a discrete (and really HOT) chip that does memory decode/timing, and this make their memory access slightly more flexible than portable Macs (which use an integrated RAM controller inside the processor module.)

The issue is if these DIMMs DON'T work, will you have any recourse with the seller? Some have steep restocking fees. Some do not accept returns. Some call you a liar and tell you to go away.

you're welcome.