Why not just use a FET?
Because the best reson to use an optoisolator is the total electrical isolation between your motherboard and your circuit. Now, if you are a serious electronics gawd, feel free to use whatever interfacing strategy you like. But the opto is cheap and gives your expensive mobo a lot of protection from things like short circuits. I recommend a 4N27 optoisolator. This item should cost about $0.35 from an electronics discount house like Hosfelt. The basic theory of operation is that the HDD activity signal activates an infrared LED. This is coupled optically to a phototransistor, so that the transistor conducts when the HDD activity signal is ON.
