Cable select on 40 wire cables: Using the 40 wire cables the primary unit (master) will be in the middle, the secondary unit will be the outer connector. If you connect only one unit it is recommended to connect it to the outer connectors. This prevents data integrity problems due to signal reflection. Most drives and BIOSs can handle a standalone secondary unit (Slave). If not do not connect the drive to the middle connector but jumper it to be the primary unit (Master).