Q. What Memory and Drives fit my MacPro?

There have been several models of Mac Pro over the years, and there have been four types of memory modules used.

MacPro1,1 2,1 and 3,1 Mid 2006 – Early 2008 Aluminum tower
The Mac Pros that use FB-DIMMs (either 667 MHz or 800 MHz) require heat sinks with extra large fins to comply with Apple’s cooling critera. FB-DIMM stands for Fully Buffered DIMMs, which are Error Correcting (ECC) by definition.

These Mac Pros have two memory riser cards, each card has four memory sockets, numbered from the bottom up 1,2,3 and 4.  The memory riser cards are identical and interchangeable, but are conventionally called “A” and “B”.  RAM in these machines must be added in pairs of matching modules – although you don’t have to match one pair to another.  There is a small speed up available when installing four maching modules, for 256 bit memory access.

The  installation for RAM modules for these machines is to install the RAM in order from the largest to the smallest modules, in these pairs:

1) Riser A, Socket 1 & Riser A Socket 2
2) B1 & B2
3) A3 & A4
4) B3 & B4

MacPro4,1 and MacPro5,1, Early 2009 to 2012 Aluminum Tower

The later generations of Mac Pro use DDR3 memory, either DDR3-1066 MHz or DDR3-1333 MHz.  These are ECC DIMMs without the large heat sinks, and they have a thermal sensor on board the the module. In general, you can use compatible DDR3-1333 ECC DIMMs in both the 1066 MHz and 1333 MHz machines.

The machines are configured with either four memory sockets (single CPU machines) or eight memory sockets (dual CPU machines).  The way the sockets are arranged is a little confusing. The machine will implement dual channel memory access, or triple channel memory access, which is faster than dual.  For triple-channel, you would install matching RAM modules in sockets 1,2 and 3  (and 5,6 and 7 for 8 socket machine) and the top socket in each bank (4 and 8) must remain empty.  As soon as you install a fourth module in the top socket, it falls back to dual-channel, and if that module is not a matching pair with socket 3 (or 7) the the machine will drop to single channel memory access, which is about 6% – 8% slower in overall performance than dual channel.  So it pays to plan ahead for memory upgrades – don’t succumb to the temptation to put a smaller existing module “to good use” by installing it into the top socket, you could drop the performance of the machine.

Installation order (eight socket machines in brackets):

Sockets 1,2   (5,6)
Sockets 1,2,3 (5,6,7)  <fastest configuration
Sockets 1,2,3,4 (5,6,7,8)

Registered vs. Unregistered.  Apple ships 1333 MHz 8 GB modules as ECC Registered modules, while their 2 GB and 4 GB modules are Unregistered. This means you cannot mix 8 GB modules with lower value modules because you can’t mix Registered RAM and Unregistered RAM in the machine at the same time.  The safest bet is to remove all existing RAM when installing 8 GB module.

MacPro Late 2013 Cylindrical Black machines

The 2013 redesign of the MacPro introduced faster RAM (DDR3-1866 MHz) and did away with spinning hard drives entirely, relying solely on SSD drives, which are a proprietary SSD card format specific to Apple.  Additional drive storage has to be added through Thunderbolt 2 or USB 3.0 enclosures.

These machines have four memory sockets, however RAM performs fastest in the MacPro 2013 in Triple Channel access mode, with three identical RAM Modules and the forth slot remaining empty.

Lists of RAM and drive compatibility for all Macs