A hard disk drive (HDD) is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order.
HDDs are a type of non-volatile storage, retaining stored data when powered off. They are used either as the primary or secondary storage device in a computer and are commonly found in the drive bay and are connected to the motherboard via an Advanced Technology Attachment (ATA), Serial ATA, parallel ATA or Small Computer System Interface (SCSI) cable, among other formats.
Hard disks are flat circular plates made of aluminum or glass and coated with a magnetic material. They can store terabytes (trillions of bytes) of information and data are stored on their surfaces in concentric tracks. A small electromagnet, called a magnetic head, writes a binary digit (1 or 0) by magnetizing tiny spots on the spinning disk in different directions and reads digits by detecting the magnetization direction of the spots.
It is important to note that a hard disk drive and hard disk are not the same things, but they are packaged as a unit and either term can refer to the whole unit.
