First of all, an optical drive can be a CD, DVD, or BluRay drive. The button isn't invisible, it's just hidden in plain site by design. The previous pictures show its location, but here is one more. This is a picture of the top of a 660 showing 2 drives.
What is difference between an osteon and osteocyte? An osteon is the functional unit of cortical bone. Osteons contain osteocytes, which are cells that produce bone matrix.
(3)Osteocytes (in lacunae)- maintains the bone matrix and are stress/strain sensors.  (4)Canaliculi-hairlike canals that tie all osteocytes in an osteon together so they all get nutrients and dispose of waste.  note: lamellae fills the gaps between forming osteons and helps resist twisting in the long bone(s).