The platters, spindle motor, heads, and head actuator are inside the drive, sealed from the outside. The HDA is rarely opened, except by professionals. On the outside are the logic board, bezel, and mounting equipment. Below, I will describe each of these components. The platters are the disks inside the drive.
Platters can vary in size.
Most drives have two or more platters. The larger capacity drives have more platters. They are usually made of an aluminum alloy so that they are light. Basically, this is glass with enough ceramic within to resist cracking. This glass technology is taking over aluminum in the hard drive industry.