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 at an earlier stage of development (when they were called osteoblasts) formed the bone matrix. Osteocytes have long processes that extend out to touch neighboring osteocytes. The processes of osteocytes near the central canal also extend to the central canal.