918 Upgrade and Maintenance Manual  TX300 S7 / RX350 S7
Cabling
16.2.8 Power cabling with 8x 3.5-inch HDDs, 2x 3.5-inch 
HDD extension box and backup drive (solution 2)
Figure 643: Power cabling with 8x 3.5-inch HDDs and 2x 3.5-inch HDD extension box
I Alternative power cabling solution with power cable T26139-Y3939-
V202 for configurations involving an HDD extension box.
System Board D2949
PSU CONTROL 
PWR
Power Backplane
X25
X21
X20
X23
PWR HDD / ODD
4x 3.5-inch HDD 
Backplane 1
X9
X10
X7
4x 3.5-inch HDD 
Backplane 2
X9
X10
X7
Drive Bays 2-3
2x 3.5-inch
HDD Extension Box