DrivePowerState is a tiny (<50KB download) portable program for monitoring how much time hard drives spend powered. This might help you spot battery life issues on a laptop, for instance, if Windows isn't powering a drive down as often as you'd expect.
Launch the program and it detects and displays your drives. Details include their serial number, volume name, mount point, power state, idle time and powered time (both zero initially) and cycles.
By default the program refreshes its display and figures every minute. When we checked later, it told us our internal drives C: and D: were powered, had 0:00 idle time, 0:19 powered time, and therefore a duty cycle of 100% (they were on all the time). An external drive H: was sleeping, had an idle time of 0:12, a powered time of 0:07, and a duty cycle of 40% (seems like there's a little rounding there, but you get the idea).
Clicking the Hide button minimises drivePowerState to the system tray, where it carries on logging in the background.
By default the program only displays the current power state and measurements, but click Settings and you'll find options to keep summary logs (just the totals) or continuous logs (every sample taken).