The WD Easystore external drives do not have a power button to turn the unit off and on, A possible solution would be to plug the power brick in to a power strip with a switch.
(I think that another version of this drive sold by other retailers - the WD Elements external drive - does have a power button on the back. However, this button looks to be an electronic switch. So even that drive will remain connected to power when turned off with the switch.)