The ribbon cable socket on the back of the drive has two rows of pins. In one of the rows, a pin is missing.
ribbon cable socket
missing pin
3.Align the connector with the socket so that the row in the connector with the blocked hole lines up with the row in the socket with the missing pin.
missing pin
hole
Make sure the holes fit over all the pins and then push the connector onto the pins.
Caution
If you do not correctly align the holes with the pins, you could severely damage your hard disk drive when you push in the connector.