Epson Equity 320SX PLUS - Manual (Page 142)

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, as shown below.
Make sure the holes fit over all the pins and then push in
the connector.
If you do not correctly align the holes with the pins
you could severely damage your hard disk drive when
7-14 Installing and Removing Disk Drives