Is the 1TB fusion drive two physically separate drives - one a 128GB and the other comprising the remaining storage to 1TB? Or, is it a 1TB drive and a separate 128 SSD? Or, one physical drive equaling 1TB, with two different properties? If it is one drive, does it appear in finder as two different volumes? Also, if they are one drive, doesn't the SSD portion stand to fail if the HD fails?