90V AC on hard disk?!?

[Major Scott]

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.

 

[Major Scott]

Something is effed up.

It works though.

I've had a similar voltage out of the composite output of a VCR. That's the trouble with these double insulated things that aren't earthed.

 

[Major Scott]

Any double insulated SMPSU has filter caps from each mains pole to the
equipment 'ground' - they form a capacitive voltage divider (Xc + Xc).

That sounds rather strange to me. Especially where delicate electronics are involved. Imagine I connected the USB hard disk, while it was powered up, to the computer, and the USB plug's grounded chassis managed to touch one of the signal contacts.

 

[Major Scott]

Could be time to try another PSU ??

If Ian is correct it won't help. I'd have to find one in my huge box of them with the right output then change the plug. I'll just be careful with it.