Originally posted by: Blain
Odd that the connectors for both are 40 pins.
It provides backwards compatibility. Otherwise, older equipment would need its own cable (which would become harder and harder to buy over time) separate from newer equipment.
@OP: The 80-wire cable has extra wires to reduce crosstalk (they are all connected to ground). This permits operating at higher speeds without unacceptably high error rates. 40-wire cables are, as stated, fine for optical drives and will work for hard drives provided you're willing to give up performance.
Given that new IDE ribbons cost about $5, I'd just buy a new cable. Or rearrange the drives.