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.
Osteons, or the structure that forms compact bone, surround these canals and are the outer component of the haversian system. The terms osteons and haversian system are often used interchangeably. Haversian systems are units of structure within mature bone. The systems run parallel to the long axis of bones. Each cylindrical unit within compact bone is an entire haversian system. The system includes lamellae, osteocytes, lacunae, canaliculi and a haversian canal. Lamellae are the thin layers that compile and eventually form bone. They contain collagen fibers that are arranged in a parallel array.