HDDs

A hard disk drive (HDD) is a non-volatile storage device that stores digital data on magnetic surfaces of spinning platters. It comprises one or more rigid disks, read/write heads on an actuator, a spindle motor, and an interface to connect to a computer. Data is written by magnetizing regions on the disk and read by sensing the magnetization as the platters rotate.

Data organization and access: platters are divided into tracks and sectors; a controller maps logical blocks to physical locations. The actuator positions the heads; performance depends on rotational speed (RPM), transfer rate, and cache. Consumer drives commonly run at 5400–7200 RPM; enterprise models may exceed these speeds. Cache memory, typically DRAM, buffers data.

Form factors and interfaces: the main sizes are 3.5 inches (desktops/servers) and 2.5 inches (laptops). Interfaces include SATA for consumer drives and SAS for enterprise. External USB enclosures are also common.

Strengths and limitations: HDDs offer large capacity at low cost per gigabyte and are suited for sequential workloads, backups, and archival storage. They have higher latency and lower random I/O performance than SSDs, and are sensitive to shock and noise.

Reliability and lifecycle: modern drives include SMART health monitoring. Lifespan is typically several years but varies; failures can arise from bearing wear, head crashes, or electronics. Regular backups and, where appropriate, redundancy help protect data.

History and trends: HDDs were introduced in the 1950s by IBM and have evolved toward higher areal density. Technologies such as perpendicular recording have increased capacity; newer approaches like SMR and HAMR continue to expand density. Despite rising SSD use, HDDs remain common for bulk storage.