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Data storage devices are used to store digital data in a machine-readable format, fulfilling one of the core functions of a computer.

When an operating system is installed onto a computer, or a program or application, and when a file is saved, they are stored on a secondary storage device like a hard drive. As opposed to primary storage, known as main memory, RAM, ROM, or cache, secondary storage is used to permanently store computer software and data.

There are three types of secondary storage devices: magnetic, solid-state, and optical.

Magnetic storage devices include hard disk drives, which use a magnetic field to magnetize sections of the disk to store data. Modern storage devices of this type tend to be large and inexpensive. Solid-state devices use flash memory to store data and tend to be small, fast, but expensive for the amount of data they can hold. Optical storage devices use a laser to read data from a CD, DVD, or Blu-ray disk. While data can also be written to the disk, it is usually permanent.

Hard drives can be compared to filing cabinets. They are used to permanently store the computer's operating system, installed programs, documents, photographs, music, and other data. When you click to boot up a web browser, a word processing program, or a game, the software for that application is loaded from the hard disk into the computer's primary memory (RAM), so that it can be used. The size or capacity of a hard drive is usually measured in gigabytes or terabytes, although older drives were measured in megabytes. The disk spins at 7200 rpm, although some drives are slower.

The disk connects to the computer's motherboard using a SATA cable. Computer systems usually come preinstalled with a hard disk drive, also known as an internal drive because it is installed within the computer casing. A hard drive must be formatted by the operating system before it can store data. Inside the hard drive is a stack of double-sided disk platters with an actuator arm that contains an electromagnetic read/write head that hovers just above the surface of the disk. As the disk platters rotate, the actuator arm moves back and forth, positioning the head on the correct track or sector of the disk in order to read or write the necessary data.

Hard disk drives that are not installed within the computer casing are known as external hard drives. Designed for portability, they are smaller and they generally plus into a computer's USB port.

Solid state drives perform like hard drives but are faster and more expensive. SSDs are made of non-volatile NAND flash memory, a type of memory that retains data even if the power is turned off. SSDs have no moving parts, and are used in smaller laptops and notebook computers, as well as in some tablet devices, where large amounts of data storage are not required.

SSD drives can hold more than two terabytes but large capacity SSDs are expensive. Because they are much faster than magnetic storage devices, some modern desktops come with an SSD installed.

Not used so much anymore, DVDs and Blu-ray drives were popularly used to store movies, while CDs were once commonly used for movable data storage and as data backup. The surface of a CD, DVD, or Blu-ray disk is marked with pits, which is how the data is encoded onto the disk. Whenever a pit is encountered, a laser beam is not reflected and is interpreted as a 0. If there is no pit, the laser beam is reflected and interpreted as a 1.

Laptops, tablet devices, and digital cameras are often shipped with built-in memory card readers, the most common of which is the SD card, which itself may be full-sized or a Micro SD card. Adaptors are available to allow the use of a Micro SD card with a standard SD reader or for use through a USB slot.

Also known as memory sticks, USB flash drives are small, and they plug into the USB port on a computer or laptop, allowing files to be copied to them for easy transport of data from one device to another. USB drives come in a variety of shapes and capacities.

Network attached storage (NAS) drives are used to store and backup files to a central point on a network. All of the machines on the network can access files stored on the NAS drive.

Some vintage computers used tape cassettes or floppy disks for storage, and many of these vintage machines are still in use by hobbyists. Other, more short-lived types of storage have also been used throughout the years and would be appropriate topics for this category, as well as those that are commonly in use today.

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