Hard Drive Activity Meter

This circuit is designed as a replacement for the standard HDD activity LED and offers an indication of the amount of activity by lighting a row of LEDs. The higher the HDD activity, the more LEDs light. It does not give a reading of actual Mb/s, just how hard the drive is working.

http://www.bit-tech.net/modding/2002/06/23/hdd_activity_meter/1