The drives are on a timer. When there is not any activity on a drive it starts the timer. If still no activity on the drive at the end of the timing cycle the drive gets put to sleep or spun down. This conserves power and extends the life of the drive. When you call for the drive as in opening a file or saving something to the drive it will wake up or spin up.
The timers can be changed or disabled. However I personally think it's best to let the drives spin down when not being used. It does take a second or two to respond to a call when spun down. A small price to pay for less power drain and longer life.
Here's how you do it.
Control panel > Hardware and Sound > Power options > Edit plan settings
One the screen it will show a statement that says > Change advanced power settings. Click on that.
A box pops up with all the settings for your system as far as power options. Click on the + next to Hard Drives and it shows the current timer setting. Click on that and select the amount of time you want to pass with no activity before it spins down the drive.
You can also look at the other settings and adjust them to see how your system performs. But be careful that you don't get too carried away.
