If you have a 12GB RAM system, can you add a 16GB stick and place it in the #4 slot and have it work, equating 28GB?

In theory, it ought to work, yes, but, a44andrew, as you mentioned, apple prefers same pairs for Ram, and not any RAM will do. It has to be Fully Buffered ECC ram, some mac Pro's like Ram in pairs, some are not quite as fussy.....

I think this diagram from Apple says it all: this is the one I used when upgrading my ram, I have a 2008 mac pro

so I'll stick with this because it works

http://support.apple.com/kb/ht4433

JB

The specific scenario you cite does NOT work, in a fairly spectacular way. But not for the reasons you suggest.

The only 16GB modules available today are Registered DIMMs. Registered DIMMs cannot be freely mixed with Un-Registered DIMMs -- you must choose one type or the other. The original DIMMs shipped in your Mac are Un-Registered DIMMs.

If you buy two 16GB Registered DIMMs, you could get to 32GB, but you would have to remove the three original Un-Registered DIMMs.

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Many folks prefer to buy modules from OWC/MacSales. They provide good support. Their prices can be a little higher than some others with comparable support, but they will give you a few dollars back for your used DIMMs. This somehow "feels better" and so lots of folks do it their way.

Unfortunately you would not be able to

The Mac Pro 2013 comes shipped from Apple with ECC unbuffered memory, the maximum size dimm made for ECC unbuffered memory is 8GB, so you could stick and 8GB module in the last slot, single 8GB compatible module below

http://www.ebay.co.uk/itm/8GB-1x8GB-Memory-Ram-Apple-Mac-Pro-6-1-2013-Black-all- CPUs-ECC-UDimm-/121323817375?pt=UK_Computing_Apple_Desktops_CV&hash=item1c3f7689 9f

This would give you a total of 20GB (4GB+4GB+4GB+8GB)

However I would recommend that you install the 8GB module in slot 1 and the other 4GB module in slots 2/3/4

Alternate solutions, full kits

24GB (3x8GB) ebay (121336713283) @ £159.95,

- you could always add one of your old 4GB module giving you 28GB

32GGB (4x8GB) ebay (171311216308) @ £199.95

Any modules bigger then 8GB are ECC Registered and not compatible with Ram supplied from factory Apple Machines.

Any questions let me know, I wil be glad to help

Regards, Abdul

I may receive some form of compensation, financial or otherwise, from my recommendation or link.

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