Right. I've built the circuit and it works fine on two of my PCs, but on my main PC, it doesn't. Great. When theres no HDD activity I can adjust the pot just fine so that no leds light up. I can't however manage to make them do anything at all when the HDD is active. No matter how much I adjust the pot, the damn thing won't light up I switched C2 with a 10uF cap hoping that would do something, but that didn't work either. I really am puzzled. Any ideas?