I have 8gb of RAM (8 x 1gb modules, 1gb for each slot) in my January 2008 Mac Pro. I have been running this configuration for a year now with no problems. I noticed today that "About This Mac" says I only have 4gb RAM... I look a little deeper and EVERY slot on Memory Riser B comes up as empty. So I check the riser, pull it out and reseat it... Nothing. Still only 4gb Faulty Memory Riser?