My Book Studio 2 TB Sparks iMac ??

Hi guys , I purchased a WD My Book studio 2TB from apple shop in Norwich . I was told that I could use this for saving Data and as Time Machine on our apple iMac . While trying to insert the USB into the back of our iMac I noticed small sparking ,like static ?. Works fine , just concerned , as M B Studio is self powered , would it cause trouble / damage or is the apple store representative correct in saying that is not going to cause any damage ? Is there a correct way of powering up the MBStudio , like turn it on after inserting the USB and before booting the iMac ? Please advise , thank you in advance . . .

Is that the aluminum one? Because it does conduct electricity and random sparks happen.