External HDD sparks --> USB ports

[Krinidon]

My english is above average but I hope I won't confuse anyone, so here we go:

 

Living in Greece. I have an SLI system, its a Coolermaster Storm Scout (first gen), i7, SLI, Coolemaster Silent Pro 850w PSU.

I also own like 10 external hard drives and often, I swap em on my USB ports (mostly on front). Those 4 USB ports are ending on motherboard by 2 cables with an adaptor (so its impossible to connect em in a bad way or something).

 

I figured out, that a specific drive, which is an Intenso 3 TB external HDD, creates sparks when I touch its usb connector to the metal part of USB port on front of my case.

You don't need much effort to recreate this spark and in some cases I have the impression that the flow is continuous (Raiden Mortal Kombat lol). I don't get hit by power, there is no sound, I am usually barefoot and the floor ain't wood, touching the case doesn't kill you, a testing screwdriver doesn't go on (lol).. The newest external HDD I have, its an WD 4 TB drive, needs lots of effort to recreate a spark (had to hit the 2 metal parts like 60 times to see a spark). My oldest external HDD, a WD 250GB creates sparks but the Intenso one, is by far the champ. Still, the system works PERFECTLY for more than 4 years.

 

The wall socket is grounded, and I tried to recreate it with or without my voltage regulator (bought a new one yday cause the other was quite old) + 2 surge protectors I have (voltage regulator is in the middle of em).

 

So the question is this, ok, the external HDDs don't have grounded sockets (schuko plugs), my motherboard sits on those 9 screws (don't know how they call em), all cables inside seem fine, no powered cable touches anything metal, the PSU has metal screws to the case, that metal protector for motherboards ports sits fine (its supposed to serve for grounding aswell I guess)  the case is sitting on plastic wheeled thingie. Is it normal to see a spark from a non grounded powered device? Some say yes, some say no, one drive does it easily, other doesn't and you have to struggle. I will mention finally that non powered things (like a camera, usb stick) don't create anything. I never had a blue screen or restart or crash (except a corrupted BIOS like a month ago but from another reason). 

My mobo is the 

 

http://www.gigabyte.com/products/product-page.aspx?pid=4144#ov

 

(Ultra Durable, bla bla bla, grounding protection etc etc).

[SCHISCHKA]

what is the adapter you are using?

[Krinidon]

I didn't explain it correctly, its the actual connector from the case (not using anything mine):

 

[SCHISCHKA]

so its sparking on the motherboard USB header? that would be caused by a loose connection

[Krinidon]

Its sparking on the USB ports in front/top of the case (outside). The connectors are connected properly.

 

https://www.dvhardware.net/news/cooler_master_storm_scout.jpg