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.
An osteon is also called a Haversian system. The mature bone cell is called an osteocyte. It sits in a space, called a lacuna. An osteoblast is an immature bone cell. The osteoblast is the bone forming cell.