Powering up this setup you should see the needle moving. If it turns to the wrong direction, you plugged it backwards, invert the 2 connection on the meter. From this setup, you want to adjust to the correct values. We are running 5V on the meter, and the HDD led indicator is at 3.5 Volts.
However, knowing the current of the lead will overshoot each time it is initially activated, we want to use this in conjuncture with the needle for a nice visual effect. Therefore the 5V should be placed at maximum value, leaving the 3.5V about at 3/4 of the meter range.
Click to Enlarge
Now that we have the proper needle position, a lecture on the Potentiometer leaves us the proper resistor value to use. In the case of the Radio Shack VU-meter, it was 24K ohms.
All that is needed to be done is to solder the resistor in series with the VU-meter, and manage some connector to plug into the HDD-indicator header. This depends of your computer's physical setup.
Back of VU Meter
When done, replace your HDD led indicator with the new needle HDD meter, copy large files and enjoy the show! Of course, you might want to manage a nice place in you front bezel to include the meter. If you took apart a commercial component, such as a battery tester, its bezel might come in handy. You can also mod the background image of the VU-meter for you own needs: Radioactive sign, different scale, etc.
Those who think this setup should be isolated from the motherboard might want to know that I had mine running 24/7 on my rig for 6 months without any problems so far, but as usual we cannot be held responsible for any damage or trouble encountered trying this mod.
Take a look at the VU meter in action here (3.2MB .avi)!