Hey folks... I have a new Mac Pro. Purchased it last week with 1GB RAM (Apple 512x2) and bought 4GB from OWC. All was working fine over the weekend. The 4GB arrived today (4x1GB). Here's the oddity... When I install it like this: Riser A, Slots 1 & 2: 1GB OWC DIMMs Riser A, Slots 3 & 4: 1GB OWC DIMMs Riser B, Slots 1 & 2: 512 Apple DIMMs Riser B, Slots 3 & 4: Empty then OS/X sees all 5GB and things seem to operate normally. However, per this graphic I found in my searches here, I noted that the RAM is ideally installed the other way, below. BUT, when I install as such: Riser A, Slots 1 & 2: 1GB OWC DIMMs Riser A, Slots 3 & 4: 512 Apple DIMMs Riser B, Slots 1 & 2: 1GB OWC DIMMs Riser B, Slots 3 & 4: Empty The machine does not see / recognize the RAM on Riser B. In this configuration, it reports that Riser B has all four slots empty (so it reports 3GB RAM installed.) I have repeatedly checked that everything is seated properly. Any clues what is up? Thanks all.