If you would have gone to Western Digital FAQ about their external drives, you would have seen.
Most WD external hard drives will spin down (enter into power saver mode) after 10 minutes of drive inactivity. Once the drive is accessed again, the drive will exit from the power saver mode and spin back up.
Note:
This is a permanent feature of most of our external hard drives and cannot be disabled. WD externals that include WD SmartWare 1.6.X can use the software to change and/or disable this feature.