MacPro 6.1 memory in 5.1

The RAM from the late 2103 Mac Pro would be DDR3 ECC 1.5 Volt, running at 1866 MHz

The RAM from the Mac Pro 2012 would be DDR3 ECC 1.5 Volt running at 1333MHz

The RAM from the Mac Pro 2009 would be DDR3 ECC 1.5 Volt running at 1066MHz

>> IF and only if the RAM could step down correctly to run at slower speeds, you could use Mac Pro 2103 RAM in an earlier Mac Pro (2009 through 2012).

It is a compatible type, DDR3, and the key on the RAM slot would accept it, and it would do no damage (provided you did not use a Hammer during installation) but it may or may not work. The manufacturer must have taken steps to assure that the faster RAM would also run at slower speeds with the appropriate slower timing.

None of this would work in older 2006-2008 models that require matched pairs FBDIMMs, a different type with a different key in the memory slot.

No. The memory for the MP 6.1 (late 2013) is not compatible with earlier MPs

Frequently higher speed memory will not work. Try at your own risk

Yours was the first one post I have seen on that subject.

The number of pins, voltage and DDR3 designation looks to be compatible.

There are lots of sad stories here on the forums about folks who bought RAM that appeared to be different from their original RAM in ONLY that the clock speed was faster. They plugged it into their Mac, and ...

... discovered it did not work, because it could not shift down properly to run at the slower speed. Now they own some very fine modules that work great -- but do not happen to work in their Mac.

Once you get close to 8GB modules, you run the risk of encountering different chip organization and registered vs. unregistered problems.

If you had some modules that "fell on you", you could certainly try them. But pay money for stuff that was not sure to work, and is generally not returnable easily? That is not to my liking.

some folks on eBay have no idea what they are selling.

Others are remarkably clever.

If you watch intently, sooner or later someone will say "I also tried these in my silver tower Mac pro 2009-2012, and they work great". Money back guarantee.

>> Buy those!

Would testing 2013 MP 4 GB sticks in a 2009 Mac provide and such test?

I upgraded my 2013 and still have the original 4 GB sticks. I do not want to damage either the memory or the 2009 MP

Check that processor on Intel Ark page for what memory it can support, if it is 56xx series I would give it more of a chance if the firmware code of the DIMMs and SMC can talk to each other. It is up to the processor in large part and the processor daughter tray I would think.

I think, I have read, of people (trying) to use even 1600MHz instead of 1333MHz DDR3 in a cMP.

I just tried the original memory (4x4GB) from my 2003 Mac Pro (it now has 4x8 GB) in my 2009 Mac Pro and it seems to work fine; not problems yet. It is identified as 1066 memory

So in lllaass' case, it seems his father Mac Pro late 2013 memory DOES step down and run properly at the lower speed of the 2009 Mac Pro.

Otherwise, the Error Correction Hardware would be going crazy.

I think I meant to write:

"Faster" Mac Pro late 2013 memory

... above, but got spell-corrected into something else