Low electric current felt on Macbook PRO

Hi ya'll,

I've just purchased a 2012 13" Macbook Pro this Tuesday and was incedibly excited when I got it. Everything has been going pretty swell so far until today. Previously I had backed up all my data from my last Macbook into an external had drive. Today when I pluged in the hard drive, which has it's own power cable, I felt a slight tingle under both of my palms.

It seems that this has also been a problem to other users-->https://discussions.apple.com/thread/2430990?start=0&tstart=0

However, I was not able to find a sloution to this problem. Is it normal? Will it go away? Do I have to replace my Macbook Pro?

Please help. I'm deelpy in love with my new laptop and I would hate to find out that this is a defect due to the unibody or construction.

Thanks for your time!