Epson 3865X Plus User Manual page 244

Both the diskette drive and hard disk drive sockets on the
main system board have a notch on one side. The hard disk
drive socket is closest to the front of the main system board
and the diskette drive socket is behind it. The hard disk
drive socket is also a bit longer.
Connect the diskette drive ribbon cable first. As shown
below, there is a tab on one side of the connector.
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 the pins in the socket
and then push the connector onto the pins.
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.
diskette drive connector
tab
diskette drive socket
notch
Installing and Removing Disk Drives
B-53