Solder the 47K Ohm resistor to ground and to 4N25's pin 4.

Short 4N25's pin 4 with 4049's pin 3.

Solder the HDD's activity cables coming from your motherboard. Check out your motherboard's manual, or take a look at your motherboard to get which pin is positive and which pin is ground. Solder the cable that goes to the motherboard's positive pin to 4N25's pin 1 and the motherboard's ground to pin 2. This is very important, if you solder these cables the other way, the circuit won't work.

Put the 4049 on the IC socket.

The finished product.

Download the video of the indicator in action!