Epson 3865X Plus - Manual (Page 245)

7. Now connect the hard disk drive ribbon cable. As shown
below, there is a tab on one side of the connector, just like
the diskette drive connector. The hard disk drive socket at
the front of the main system board also has a notch on one
side.
hard disk drive connector
tab
hard disk drive socket
Align the connector with the socket so that the tab on the
connector lines up with the notch in the socket. Make sure
the holes in the connector fit over all of the pins in the
socket and then push the connector onto the pins.
Caution
If you do not correctly align the holes with the pins in
the socket, you could severely damage your computer
when you push in the connector.
B-54
Installing and Removing Disk Drives