Secondary storage refers to internal or external devices that store data and applications that aren’t being used. These devices are nonvolatile and don’t lose data when powered down. Hardware content addressable memory sends a number to the CPU to access information stored in its cells. The memory can be accessed using software or hardware, the latter being faster but more expensive.
1. Hard Disk Drive
A hard disk drive is a magnetic data storage device that stores and retrieves information using an actuator to read and write to rotating platters. Data stored on a hard disk is organized into files and directories that are referenced by an operating system.
The physical size and cost of hard drives have been decreasing while data storage capacity has increased substantially. Hard disk drives are often used for backups, disaster recovery (DR) and long-term archival purposes and might be located on premises or hosted by service providers on their cloud platforms.
A portable hard drive, also known as a USB flash drive or thumb drive, is another form of secondary memory. It connects via a USB port to a computer and allows users to save and transfer data across devices.
2. Flash Memory
Flash memory is a nonvolatile, electrically erasable programmable read-only memory chip that can retain data even when the power is turned off. It is widely used in USB thumb drives, digital cameras, mobile phones and personal computers to store data.
It’s also found in medical electronic devices such as ECG machines and Holter monitors, and in the latest robotic surgery equipment like needle surgery apparatus and LaparoGuard. Flash memory is usually less expensive than secondary storage and has a higher resistance to physical shock than a traditional hard disk drive.
The biggest disadvantage of flash storage is its limited endurance, which can only be written a certain number of times before the memory cells begin to wear out. However, modern consumer devices typically incorporate a DRAM cache and a memory controller that can speed up writes and improve their longevity. The difference between the primary and secondary memory is that primary memory is immediately available to the CPU, but secondary memory is not directly accessible to the CPU and is used to store data.
3. Optical Drives
Optical drives allow you to use CDs, DVDs and Blu-ray discs for music, movies or data storage. Most also allow you to record content onto a blank optical disk, which can be useful for creating backups or sharing files with friends.
Unlike hard disk drives, which are magnetic, optical discs have electrical circuits in the memory cells that store data. High circuits indicate a 1, while low ones represent a 0.
While optical storage devices remain useful, consumers became less willing to use them when flash memory became popular, and cloud computing became widespread. Tertiary storage, also known as tier 2 storage, is a level of computer storage used for rarely accessed data. It usually involves a robotic mechanism that mounts and dismounts removable mass storage devices.
4. Solid State Drive
A solid state drive (SSD) is an internal or external non-volatile storage device that enables permanent data storage. It is commonly used as a backup or auxiliary storage device, and in some cases as tier 2 storage.
SSDs have no moving parts, which make them more rugged and durable than HDDs. However, the chips have a limited number of times they can be written before they wear out, so some file systems like ZFS for FreeBSD include a process called TRIM that informs the SSD to skip overwriting certain blocks of data.
SSDs are based on semiconductor cells that hold 1 to 4 bits of information. They are available in a variety of form factors including 2.5-inch, which fit into hard disk drive slots on desktop computers; M.2 that connects directly to a motherboard; and U.2 that connects to a PCIe slot.
5. Memory Card
Memory cards store data on portable devices like smartphones, cameras and handheld gaming consoles. They are slim and lightweight, and can be inserted into a built-in slot or by using an adapter.
Memory card reading and writing is based on the fact that transistors are either “on” or “off.” When an electrical current passes through a memory cell, it changes the ons and offs into 1s and 0s. Afterwards, this information can be retrieved.
A secondary storage device, also known as auxiliary or backup memory, is any non-volatile memory that enables permanent data storage. It stands in contrast to primary storage, which refers to a computer’s volatile memory devices, such as RAM and cache. It may also refer to external devices such as hard disk drives, optical discs or USB flash drives.