Only a quick demo of how it works, saves me having to re-cable manage.
Black wire is equal to negative, red is the positive.
Battery is just to demo it, in reality one red wire would be coming from you hard drive led wire and the other from your power button led wire on your case.
when you're powered on, the LED will be lit red. When you activate the switch as seen in the picture or the hard drive led would flicker. The LED seen above will flash yellow. You will have to try switching the two legs with the red wires as different LEDs have a more dominant color.
Maybe some one with more expertise in electronics could advise if a resistor needs to be placed in the circuit, however I haven't had any problems.
This only really works as one color of the dual color (rgb) led has a dominant color and this is the color you use for your HDD LED.
When I get some more time I will make a video and parts list of the project, however currently all you will need is a dual color LED and some wire.