A hard disk (commonly known as an HDD (hard disk drive) or hard drive (HD) and formerly known as a fixed disk) is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, the word "drive" refers to the entire unit containing multiple platters, a read/write head assembly, driver electronics, and motor while "hard disk" (sometimes "platter") refers to the storage medium itself.

The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID, hardware such as network attached storage (NAS) devices, and systems such as storage area networks (SANs) for efficient access to large volumes of data.

Hard disks record data by magnetizing a magnetic material in a pattern that represents the data. They read the data back by detecting the magnetization of the material at specified locations. A typical hard disk design consists of a spindle which holds one or more platters, onto which the data is recorded. The platters themselves are made from a non-magnetic material, usually glass or aluminum, coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.

The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called read-and-write heads that fly very close over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins.

The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early hard disks used the same inductor that was used to read the data as an electromagnet to create this field. Later versions of inductive heads included, metal in Gap (MIG) heads and thin film heads. In today's heads the read and write elements are separate but are in close proximity on the head portion of an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive

Hard disks have a mostly sealed enclosure that protects the internal mechanisms from dust, condensation, and other sources of contamination. The hard disk's read-write heads fly on an air bearing which is a cushion of air only nanometers above the disk surface. The disk surface and the disk's internal environment must therefore be kept immaculate to prevent damage from fingerprints, hair, dust, smoke particles and such, given the sub-microscopic gap between the heads and disk.

Using rigid platters and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and access and transmit it faster. In 2007, a typical workstation hard disk might store between 160 GB and 750 GB of data (as of local US market by December 2006), rotate at 7,200 to 10,000 revolutions per minute (RPM), and have a sequential media transfer rate of over 80 MB/s.

USB flash drives are NAND-type flash memory data storage devices integrated with a USB (universal serial bus) interface. They are typically small, lightweight, removable and rewritable. As of November 2006, memory capacities for USB Flash Drives currently are sold from 32 megabytes up to 64 gigabytes.

Capacity is limited only by current flash memory densities, although cost per megabyte may increase rapidly at higher capacities due to the expensive components. (USB Memory card readers are also available, whereby rather than being built-in, the memory is a removable Flash memory card housed in what is otherwise a regular USB flash drive, as described below.)

USB flash drives offer potential advantages over other portable storage devices, particularly the floppy disk. They are more compact, generally faster, hold more data, and are more reliable (due to their lack of moving parts) than floppy or zip disks. These types of drives use the USB mass storage standard, supported natively by all modern operating systems such as Linux, Mac OS X, Unix, and Windows.

A flash drive consists of a small printed circuit board encased in a plastic or metal casing, making the drive sturdy enough to be carried about in a pocket, as a key fob, or on a lanyard. Only the USB connector protrudes from this protection, and is usually covered by a removable cap. Most flash drives use a standard type-A USB connection allowing them to be connected directly to a port on a personal computer.

To access the data stored in a flash drive, the drive must be connected to a computer, either by plugging it into a USB host controller built into the computer, or into a USB hub. Flash drives are active only when plugged into a USB connection and draw all necessary power from the supply provided by that connection. However, some flash drives, especially high-speed drives utilizing the USB 2.0 standard, may require more power than the limited amount provided by a bus-powered USB hub, such as those built into some computer keyboards or monitors. These drives will not work unless plugged directly into a host controller (the ports found on the computer itself) or a self-powered hub.