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