Epson 486SX - Manual (Page 141)

If you are installing your second drive, the cable is already
attached to your other drive; use the second connector on
that cable. Connect the cable as described below; then skip
to step 5.
To connect the cable, align the cable connector with the drive
interface so the row in the connector with the blocked hole
lines up with the row in the interface with the missing pin,
as shown below. Then push in the connector.
missing pin
Installing and Removing Drives 7 - 1 9