The IDE cables are 40 pins or 80 pins(lower inductance for faster data transfer on hdd).

The connectors have key to prevent reversed connections. Some connectors have one pin plugged up. Some cables have no key on the connectors. Some cables are made for cable select. First connector is for c: and second connector is for D:.

All hdd cable connection has buffered inputs. Meaning if your cable is put in wrong, the bios can not see the hdd. Reversing the cable will correct the mistake and nothing is damaged.

This is not true when it comes to cdroms. Many cdroms or cdrw or dvdrw at ebay may have cable reversed and input circuit damaged. Even though cmos transistors can be restored by dissipating the injected charges over time. The damage may be permanent at times, if gate oxide cracked and can not be reflowed like flash transistors.

The input circuit of the primary and secondary IDE in the chipset ais also buffered. The two connectors share the same circuit. So if hdd is plugged in reversed, it will blow out the cdrom. If cdrom is plugged in wrong way it will short out the hdd but not damage it. It only damages the cdrom.

This is interesting because of the many different kinds of IDE cables, some care and understanding is important, if you should decide to build your own computers.