Not the best method, but check if you have a ground loop by taking a 4 way adapter and unwiring the earth pin in it. Plug your harddrive into this and then into your wall socket and then see if you have the buzz. If it has gone, then you will have a ground loop. Best way to sort this is with a ground lift switch, (Which ive been meaning to get now for 3 years), which can be found from most decent electrical component stores...