Once upon a time, all desktops and laptops had LEDs that would blink whenever there was hard drive activity. Somewhere along the way, PC manufacturers decided this wasn't necessary, but I disagree: If your computer is suddenly non-responsive, it can help to know if there's still drive activity. If there is, it might just be a temporary delay. If not, you'll know it's time to reboot.

There are other advantages to having a drive-status LED, of course, but precious few modern PCs offer the feature. Thankfully, there's a software solution: TrayStatus, a free Windows utility from BinaryFortress.

TrayStatus actually offers a number of monitoring features, most of them built around the keyboard.
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Add a hard-drive activity indicator to the Windows system tray - CNET