IBM System Storage Subsystem DS5020 - Manual (Page 91)

Important: Do not cable the DS5020 storage subsystem and two storage
expansion enclosures as shown in Figure 39. A storage array with the
incorrect storage topology might appear to be functional, but incorrect
cabling can potentially have unpredictable results.
DS5020
Port 2, drive channel 1
Port 1, drive channel 1
Port 1, drive channel 2
Port 2, drive channel 2
1A 1B
1A 1B
1A 1B
1A 1B
Figure 39. Incorrect cabling of one DS5020 and two storage expansion enclosures
Chapter 3. Cabling the storage subsystem 61