IBM 2104 Model DU3 - Manual (Page 33)

9. Ensure that the 2104 Model DU3 has no empty slots. Install dummy disk drive
modules if necessary.
10. Refer to Figure 12.
11. Go to “Connecting the 2104 Model DU3 to the Power Source” on page 29.
Figure 12. Reinstalling a Dummy Disk Drive Module
Installing a 2104 Model DU3 17