Epson 3865X Plus User Manual page 230

7. Pick up the hard disk drive that you removed from the
vertical mounting position. Notice the ribbon cable socket
on the back of the drive; you see two rows of pins. In one of
the rows, a pin is missing.
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 the
connector onto the pins.
missing pin
Installing and Removing Disk Drives
B-39