I have a USB hard disk enclosure, and I'm running it without it's case as I'm testing another drive with it. When I touched the hard disk with my hand and the case of the computer (which is earthed) with my arm, I felt a tickle. So I connected a multimeter to it, and the hard disk is at 90 volts AC with a short circuit current of 0.136 mA. Is this normal? Is this a good idea when working with delicate computer electronics? I suppose the ground of the USB lead will get rid of it but it seems a bit odd. The USB disk unit is powered by a cheap wall wart that came with it, rated 12V DC 1.5A.