Epson EQUITY 3865X/20 PLUS - Manual (Page 160)

5. Line up the connector with the socket so that the notched
comers are aligned.
notched corners
notched corners
Make sure the holes fit over all the pins and then push the
connector into the socket.
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.
7-16
Installing and Removing Disk Drives