With the disc drive unit displaced, we see another board which is in fact the underside of the Adaptec ACB4070 controller board.
ST506 type disc drives are just disc drives. They need a separate controller board to operate them and the ACB4070 is one of the standard controllers used with ST506 drives, as well as its predecessor, the ACB4000A.
The disc connects to the controller board via two ribbon cables, a control cable and a data cable. One is a 34-way cable and the other is 20-way but I can never quite remember which is control and which is data.