In 500 GB or higher capacity HDD's, you can frequently find these black things (see photo below). It is made from a very weird feeling material. It is not made of plastic, it has a dull glass sound when hitting something, it is not painted black iron/steel/aluminium, it is not ferrite (a magnet cannot pick it up), it is very strong, very light and very conductive.
What is this kind of material and what is it used for, shielding? I ask this because this part is not described anywhere. It seems a latest addition, does not exists in earlier lower capacity drives.