@Zing you won't believe it but using one of the usb 3.0 ports did the job... i had one usb 2.0 port left and tried it all the time. the pen drive works there, the hdds don'`t...
just tried out the blue usb 3.0 ports and the hdd has been recognized in 9 out of 10 times... (once it wasn't, but i cannot figure out why)