Hi I have an external usb drive. It`s a cheap Akasa Integral usb case with a seagate drive in it. I have two When I connect the drive to my macbook pro and touching the macbook case i feel a vibration. When I pull the usb plug out It dissapers. When I test the usb plug with a voltage screwdriver light bulb tester the bulb lights up. Is this normal? Or is it some kind of leakage from the power supply from the usb external drive case.