FloatLED

FloatLED is a lightweight Windows software utility developed to help you monitor HD activity.
Relying on a very plain and simple bar that shows drive letters, FloatLED is actually a basic application that adopts a blinking LED system to show disk activity.
Once you launch it, the program places a horizontal bar on your screen with all drive letters, but you are also allowed to choose only the items you want to monitor. What's more, you can customize the look of the bar by changing colors for both the bar and the LED notifications.
More customization options are available at a one right-click distance, letting you set the window on top, lock window position, minimize it to Tray and disable activity and switch to a vertical layout.
FloatLED doesn't hamper system performance at all and works smoothly on absolutely all Windows versions, without even asking for administrator privileges.
The package also includes a portable version, which means that you can always copy the executable file to a disk and use it on the go without prior installation. The portable app however crashes all of a sudden on Windows 7 workstations, but the installed version works just fine.
All things considered, FloatLED is quite an interesting way to keep an eye on hard-disk activity. It's easy to use and doesn't take too much space on your screen. Furthermore, the bar can be further customized to fit any desktop, either by changing colors or by enabling transparency.