Atlas HDD Led

While several years ago, all computers came with some form of indicator for the hard disk's activity, the feature has been considered obsolete and consequentially removed. Then again, the function still comes in handy in many situations, particularly when you want to determine whether your PC's non-responsiveness is a something serious or just a delay.
Atlas HDD Led is a small piece of software designed to act as a hard disk activity indicator so that you have an alert about potentially more critical problems.

Does not come with an UI, but can be accessed from the System Tray

The setup is a quick and straightforward process that entails decompressing the archive in any desired location of your HDD. Since the application does not include an interface, you should bear in mind that you can access it from the System Tray.
You should keep in mind that the app solely presents an alert about the monitoring of your volumes at first launch only. Therefore, in case you want to access it, you can simply check the icons of the tools opened in the System Tray dedicated panel.

Could use an alert system regarding the refresh rate

Granted, the app is developed with a sole purpose in mind, namely to bleep whenever the hard disk is busy reading or writing something. While this can come in handy in case of an HDD delay when the computer becomes non-responsive all of a sudden, it would be nice if it included an alert system for various parameters, such as the refresh rate, for instance.
Moreover, considering that modern computers can have solid state drives installed as well, it would have been useful if the program could let you preview the S.M.A.R.T. attributes as well. More precisely, it would be nice if it allowed you to view the current, worst and threshold values of each indicator.

A straightforward tool for monitoring the HDD activity

All in all, if you are looking for a basic application that allows you to determine the cause of your HDDs non-responsiveness, then Atlas HDD Led could lend you a hand.