Nice HOW TO from Zapwizard – This guide will show how to make a separate LED to show exactly which drive in your system is active. This modification is done without warranty, I did this to three 80 conductor cables without any problems, if you screw up don’t blame me. First off this Mod only seems to work on IDE Harddrives, the two CD-Roms I connected (one Pionner DVD, and one Lite-On CD-RW) don’t even use this pin on the IDE cable. The CD-RW LED is always on, and the DVD LED is always off. Link.
HOW TO add drive independent activity LEDs