today I tried to put in this magic LED. With partial success. I'm running a 5500/225 board and a 2 GB samsung hd. I figured out that ATA pin 39 is the LED driver and 40 is GND. So I cut both open and attached a cable to a round 3mm low-current LED (IF=3mA, a 20 mA type is lit only very dim.)
I placed it in the microphone hole above the display. It works, but it seems to indicate write access only. Further, after running HDT benchtest the LED was constantly on so I suspect the signal status works like a flip-flop and waits for a pulse to change it's state. Unfortunately I don't have an oscilloscope to check the signals. Any ideas which other lines in the ATA cable I can try?
You might say: Your bloody harddrive is so noisy, why in the world would you need a LED? Are you deaf?
The answer is: I'm not. But I prepare to replace the hd by a CF card and make room for a DVD drive.