Why do I get sparks from my usb connector when connecting it to my Mac

I have an external hard drive with its own power. When I touch the usb cable against the case of my Mac I get little sparks. I have tried it on a PC but don't seem to get the same sparks

Why might this be?

Update:

The hard drive is using it's own power, so an earthing fault seems possible, except that it happens with an external hard drive and Mac Mini at home, and a different external hard drive and 2 separate Mac Pros at work , so an identical fault seems less likely

Update2:

A voltmeter shows a voltage difference, occasionally over 20v but it disappears too quickly to be sure.