Disk storage

Disk storage (also called drive storage) is a data storage mechanism that records data onto a rotating disk using electronic, magnetic, optical, or mechanical changes to the surface layer. A disk drive is a device implementing such a mechanism.

Major types of disk storage include hard disk drives (HDD), which contain one or more rigid platters; floppy disk drives (FDD), which utilize removable magnetic media; and optical disc drives (ODD), which use laser technology to read and write to optical media such as CDs, DVDs, and Blu-ray discs.

In modern computing, disk storage is increasingly complemented or replaced by solid-state storage technologies like solid-state drives (SSD), though HDDs remain widely used in data centers and mass storage systems due to their lower cost per gigabyte.

(The spellings disk and disc are generally used interchangeably, though standard usage prefers disk for magnetic media and disc for optical media, except where trademarks dictate usage, such as the Compact Disc logo.)

Six hard disk drives showing various form factors

Three generations of floppy disk drives (8-inch, 5.25-inch, and 3.5-inch)

An internal optical disc drive

History

Audio and video recording originally relied on analog disk methods, such as gramophone records and the Laserdisc. In the 1950s, digital disk storage was introduced to computing systems.

The first commercial digital disk storage device was the IBM 350, shipped in 1956 as part of the IBM 305 RAMAC system. Developed to provide random-access storage, disk drives complemented the high-density sequential access offered by tape drives.

Over decades of innovation, disk drives became the primary secondary storage mechanism for personal computers and enterprise servers. While solid-state storage has replaced rotating disks in most consumer electronics due to speed and durability advantages, disk drives remain a cost-effective solution for high-capacity enterprise storage and data archiving.

Fundamentals and Mechanics

Disk storage devices organize data into logical structures on physical media.

Geometry and Organization

Data on modern disks is stored in fixed-length blocks, typically called sectors.

• Platter: The circular recording disk coated with a recording medium.
• Track: Concentric circular paths (or a continuous spiral) on the surface of the disk where data is written.
• Sector: The smallest addressable block of data on a track. Legacy hard drives used 512-byte sectors, while modern Advanced Format drives use 4,096-byte (4 KB) sectors. Optical media typically use 2 KB sectors.
• Logical Block Addressing (LBA): A abstraction scheme where blocks are referenced by a linear index, simplifying physical addressing across host interfaces.

Rotation Speed and Track Layout

Comparison of disk storage track layouts: concentric tracks (left) vs. continuous spiral track (right).

The mechanics of a disk drive involve disk rotation and head positioning:

• Constant angular velocity (CAV): The media spins at a fixed rotational speed, measured in revolutions per minute (RPM). Data passes under the head faster on outer tracks than on inner tracks. This approach is standard in hard disk drives and floppy drives (e.g., 5400 RPM or 7200 RPM).
• Constant linear velocity (CLV): The media adjusts its rotational speed based on head position to ensure data passes under the head at a constant rate. This is common in optical disc drives (such as audio CDs).

Track layouts vary by application:

• Concentric tracks: Used by HDDs and FDDs to optimize random access performance.
• Spiral tracks: Used by optical media (and legacy vinyl records) to optimize continuous sequential access for streaming audio or video.

Interfaces

The disk interface serves as the protocol and physical connection between the storage device and the host system. Common interfaces include:

• Consumer/Desktop: Parallel ATA (PATA/IDE, legacy), Serial ATA (SATA), and USB (external drives).
• Enterprise/Server: SCSI (legacy), Serial Attached SCSI (SAS), and Fibre Channel (FC).

Comparison with Solid-State Storage

With the rise of flash memory and solid-state drives (SSDs), disk storage faces distinct trade-offs:

Feature	Disk Storage (HDD/ODD)	Solid-State Storage (SSD)
Access Latency	High (limited by mechanical seek time and rotational latency)	Low (no moving parts)
Throughput	Moderate (sequential speeds restricted by physical rotation)	High (multi-channel flash access, NVMe)
Durability	Vulnerable to mechanical shock, vibration, and magnetic fields	High shock resistance; no mechanical wear
Cost per Terabyte	Significantly lower (ideal for mass storage and archiving)	Higher (though decreasing over time)

See also

• Disk array
• Disk drive performance characteristics
• Disk read-and-write head
• Magnetic storage
• RAID
• Solid-state drive