revoSleep

revoSleep is a handy little tool that offers users the possibility to send their hard drives to sleep straight from its GUI.
It is addressed mainly for tech-savvy users, as switching to sleep mode very frequently might lead to damaging it.

Minimalist looks

The main window of revoSleep is minimalist, as it only displays all the detected hard drives on your computer. Moreover, when you launch it, it automatically goes to the system tray and you need to double-click its icon in order to view its GUI.
At this point you can view all the hard disks currently attached to your computer, along with several options for each of them.

Send HDDs to sleep

When you want to view all available operations for each hard disk, you can click on the 'Show details' button. You can send it to sleep, put it offline or deactivate it.
Nonetheless, it needs to be mentioned that you might come across an issue when deactivate one of your hard disks, since this might wake it up again. This is why it is best to only use the Offline function in conjunction with the Sleep one.
This way, you can make sure it is not woken up again by accident.

Supports command line parameters

Users who have more advanced technical skills might prefer using Windows' Command Prompt window, as this usually saves a lot of time.
Using the proper parameters, they easily send specific hard disks to sleep or wake them up without accessing the main window of revoSleep.

Conclusion

All in all, due to revoSleep, you can send individual hard disks to sleep or put them offline without a lot of effort or wasted time.