Can I use different size memory in the empty 4th slot I have. As of now I have three of the slots filled with 4gb of ram in each totaling twelve. I am wondering if I can put and 8gb ram or larger in the empty fourth slot.
Mac Pro, OS X Yosemite (10.10.5), currently has 12gb ram 4*3
Yes. No.
You can mix sizes, but you cannot mix Registered with Unregistered. At this writing, all 16GB modules are registered.
One unusual case if if you expect to go above 32GB in the foreseeable future. If you do, buy one or two 16GB modules now, and set aside the ones you have been using (or sell them).
If you are installing more RAM or in a different configuration than Apple recommends, be certain to buy from a Vendor who knows Macs, and can assure you:
"It works in your Mac,
or your money Back."
In the US, I suggest these vendors:
Data Memory Systems
Crucial
Other World Computing (mac sales)
When you get your new RAM try it in your Mac all by itself. THEN try mixing. This separates 'works in your Mac' from "plays nice with others'.
thank you for your advice. I really appreciate it. I was going to buy the exact stick that came with my mac pro but an 8gb for the empty slot. But i hear that doing that will reducememorybandwidth in half from a 4gb to a 8gb stick.is that true? is it better to just purchase all the same sizes?
There is very little decrease in memory speed when either mixing sizes or not using three channels
http://blog.macsales.com/22745-mix-and-match-more-memory-faster-mac-pro-2
lllaassis right. There is so much caching in these systems that the decrease in speed from mismatches is nearly negligible.
The late 2013 Mac Pro does not benefit from trios. If you accidentally end up with pairs, the pairs will interleave for a very modest boost. This is NOT a requirement. Do not lose any sleep over NOT doing that -- the difference is very small and probably not seen in the real world (outside of carefully-crafted memory tests).
The previous 2009-2012 Mac Pro was optimum with trios (on each side), and different again from the 2006-2008 Mac Pro that REQUIRED pairs in all positions.
Mixing Ram sizes in my late 2013 Mac Pro
