Hard disk drive
A hard disk drive (HDD) is a non-volatile data storage device that stores and retrieves digital information using rapidly rotating magnetic platters coated with a magnetic material. HDDs are a type of electromechanical storage and have been the dominant secondary storage device in personal computers and servers for decades. They are characterized by their relatively low cost per unit of storage compared to solid-state drives (SSDs), though they are slower and more susceptible to physical shock.
History
The first hard disk drive was introduced in 1956 by IBM as the IBM 350 Disk Storage Unit, part of the RAMAC (Random Access Method of Accounting and Control) system. It consisted of fifty 24-inch platters and stored 5 megabytes of data. Over subsequent decades, storage density improved dramatically, driven by innovations such as giant magnetoresistance (GMR) read heads, perpendicular recording, and shingled magnetic recording. By the 2000s, consumer HDDs routinely held hundreds of gigabytes, and 3.5-inch desktop drives reached capacities of several terabytes.
Features
A typical HDD contains one or more rigid platters coated with a magnetic layer, mounted on a central spindle driven by a motor. A read/write head on an actuator arm floats nanometers above the platter surface, reading and writing data as the platters spin at speeds of 5,400, 7,200, or 10,000 RPM in consumer models. Key metrics include data transfer rate, access time, and rotational latency. HDDs are available in common form factors: 2.5-inch (laptops and external drives) and 3.5-inch (desktops and enterprise storage). Interface standards include SATA, SAS, and Fibre Channel.
Use and decline
From the 1980s to the early 2010s, HDDs were the primary mass storage device in most computers. However, the rise of NAND-based SSDs, with faster speeds, lower power consumption, and greater durability, has led to their replacement in many consumer and enterprise applications. HDDs remain widely used for bulk data storage in data centers, external backup drives, and applications where cost-per-terabyte is critical.