Do I install the new pair in DIMM Riser B/DIMM 2 and DIMM RIser A/DIMM 2?
No. According to Apple*, you’ll have to pull one current DIMM from one riser to install both current DIMMs in the same riser, and the new pair in the (now empty) riser.
*) For those official DIMM installation instructions, check out Apple’s KB article
Memory (FB-DIMM) Replacement Instructions for the Mac Pro (Early 2008). Specifically, it recommends that ‘
DIMMs must be installed as pairs of identical size and type, from the same vendor. [In the illustration below], like-colored DIMMs must match.’.
PS, this KB can be found in Apple’s
Mac Pro Support section.
