Brother HL-3400CN Series User Manual page 217

USER'S GUIDE
5-60
5. Fit the 4 HDD pins into the 4 holes of the main controller board, and
secure the pillars with the screws provided from the rear of the main
controller board.
Fig. 5-79 Set the HDD
6. Connect the flat cable to the main controller board.
Fig. 5-80 Insert the flat cable