With only one CPU installed on the SYS-6028R-WTRT, only two of the 6 PCI-E slots from the two riser cards will work. The right-hand side riser card (RSC-R2UW-2E8R) has both slots coming from CPU2 and both the slots on this riser card will not work. For the right-hand side riser card (RSC-R2UW-4E8), it has two slots coming from CPU1 ( SLOT 3 and SLOT 4) and two slots from CPU2 (SLOT1 and SLOT2), and therefore, only two out of the four slots will work.