Atlas Hard Drive LED!

DriveGleam is a free app that installs a handful of activity indicators as icons in your system tray, including a CPU usage, a RAM usage, virtual RAM usage, as well as an icon that displays hard drive activity for each of your partitions.

DiskLED is a flexible hard disk and general system activity indicator system tray applet.

This is a hard disk activity monitoring tool. It displays the activity of the disk in the same manner as PC HI-DISK led does..Useful when PC HI-DISK led is not in sight or is missing. It's free to try, but in demo mode you are just prompted to register and nothing more.