Most of the enclosure is made out of a metallic alloy however the main part of a hard drive, which stores the data magnetically, is called the platter. In a hard drive there are normally several platters. Platters can store data on both sides requiring a head (reader) for each side.
The core of the platters are typically made using an aluminum or glass and ceramic substrate shaped into a disk. Then the disk is coated with a thin layer of a mostly nonmagnetic metallic alloy and on top of this is a protective carbon-based overcoat. Both of these layers are applied in the same process which is called sputtering. The next layer of the disk is a nanometer thin polymeric lubricant applied by dipping the disk into a solvent solution. The surface of the disk needs to be flawlessly smooth so that the disk head can rapidly move over the surface.