AC voltage leaking from external hard drive USB housing

After getting a bit of a zap from my USB cable and feeling this weird little vibration, I took a measurement and found about 12 volts of AC on the cable jacket relative to my PC case.
To give some context, this is the cable connecting an external WD drive to my computer. I measured the voltage while the drive wasn't actually plugged into the PC. Aside from the USB connection, the drive uses its own dedicated power supply.
It seems like this might be a known issue with certain WD drives: http://tech.iprock.com/?p=5696 but honestly, it doesn't feel safe to me, especially since my motherboard has started acting up lately... And since my wall outlet isn't properly grounded, I'm not sure if grounding would even fix anything.

Has anyone else dealt with this before? Is this somewhat normal, or should I be worried? Any advice?

It looks to me like your outlet lacks any kind of grounding—and honestly, probably doesn't have a neutral connection either. You likely wouldn't run into this mess if you were using a properly wired outlet. I dealt with a nearly identical headache once with my laptop; the chassis and ports would practically zap me whenever I plugged it into an ungrounded outlet at my old apartment, even though it worked perfectly fine in other rooms. Part of the issue was that my power adapter didn't explicitly state whether grounding was mandatory. If you look closely at the image here, you can see those two concentric squares indicating the configuration.
http://img.diytrade.com/cdimg/133561...op_charger.jpg