Disks

Disks are non-volatile data storage devices that retain information without power. The term covers several technologies, most commonly magnetic disk drives, optical discs, and solid-state disks. While all disk types store bits persistently, they differ in how data is recorded, accessed, and where they are typically used. Disk storage remains a foundational form of secondary storage in computing, complementing faster solid-state memory and volatile system RAM.

Magnetic disks, or hard disk drives (HDDs), use spinning platters coated with a magnetic material. Data is written and read by magnetic heads mounted on an actuator that moves over the rotating disks. Information is organized into tracks and sectors, enabling random access. HDDs offer high capacity at low cost per gigabyte and are common for operating systems, applications, and bulk data. Performance depends on platter speed (rpm), seek time, and sustained throughput. Interfaces include SATA, SAS, and, in enterprise systems, other high-speed buses.

Optical discs, such as CDs, DVDs, and Blu-ray discs, store data in tiny pits read by a laser. They are portable, inexpensive, and durable for distribution and long-term archival storage under appropriate conditions. Capacities range from hundreds of megabytes for CDs to tens of gigabytes for Blu-ray. Optical media are typically slower to access than magnetic disks and are frequently used for software distribution, media, and offline backups.

Solid-state disks (SSDs) use flash memory with no moving parts. They offer fast random access, low latency, and high I/O throughput, making them well suited for operating systems, databases, and performance-critical workloads. Form factors include 2.5-inch drives, M.2 modules, and PCIe-based cards, with interfaces such as SATA and NVMe. SSDs generally cost more per gigabyte but provide superior endurance and reliability for many tasks.

In practice, disk storage is managed through partitions, file systems, and, for redundancy, RAID or equivalent schemes.