Rear backplane: 4 x 2.5-inch SAS/SATA
The following shows the cable connections for the rear 4 x 2.5-inch SAS/SATA backplane if installed.
From To
Backplane 4: SAS 32i RAID adapter on PCIe slot 2: C3
Backplane 4: PWR Riser 1: PWR1, PWR2
Connections between connectors: 
1  ↔  1 ,  2  ↔  2 ,  3  ↔  3 , ...  n  ↔  n
Figure 44. Cable routing for the rear 4 x 2.5 SAS/SATA backplane
100 ThinkSystem SR650 V2Maintenance Manual