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.
Osteocytes are osteoblasts which have become trapped inside of the bone matrix. Once an osteoblast creates the new bone around itself, it is trapped and can no longer move or form bone; this is how an osteocyte is created.