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A secondary storage device refers to any non-volatile storage device that is internal or external to the computer. It can be any storage device beyond the primary storage that enables permanent data storage.
A secondary storage device is also known as an auxiliary storage device, backup storage device, tier 2 storage, or external storage.
Secondary storage devices primarily refer to storage devices that serve as an addition to the computer's primary storage, RAM and cache memory. Since they always consist of non-volatile memory, they allow the user to permanently store data on them.
Typically, secondary storage allows for the storage of data ranging from a few megabytes to petabytes. These devices store virtually all programs and applications stored on a computer, including the operating system, device drivers, applications and general user data.
They are used for a variety of purposes ranging from backup data used for future restores or disaster recovery, long-term archiving of data that is not frequently accessed, and storage of non-critical data in lower-performing, less expensive drives.
The fundamental characteristics of secondary storage are high capacity and low cost, although speed, reliability and portability might also be important. Longevity and long-term accessibility may also be an issue. For example, a lot of data from the previous decades that has been stored on magnetic tapes is now practically unusable.
Secondarily stored data might not be under the direct control of the operating system. For example, many organizations store their archival data or critical documents on secondary storage drives which cannot be accessed by their main network to ensure their preservation whenever a data breach occurs.
Since these drives do not interact directly with the main infrastructure and can be situated in a remote or secure site, it is unlikely that a hacker may access these drives unless they’re physically stolen.
Most of the secondary storage devices used to be internal to the computer such as the hard disk drive, the tape disk drive and even the optical storage drive (CD-ROM, Blu-ray, and DVD) and floppy disk drive.
However, today, many secondary storage drives are frequently external, especially since the introduction of USB flash drives and plug-and-play devices. Also, many secondary storage devices are now virtual devices residing on third-party cloud servers hosted by many services such as Dropbox, Google Drive, Amazon Web Services (AWS), or Microsoft Azure. Cloud repositories are particularly used by companies that embrace the software-as-a-service (SaaS) model.
Although many forms of backup storage such as tape drives and floppy diskettes have been long abandoned, secondary storage devices include:
Solid-state drives (SSDs).
Hard disk drives (HDDs).
USB flash drives.
Zip and Jaz drives.
Secondary drives are assigned a letter from D: onwards by the modern operating systems. That’s because historically, in old DOS and Windows operating systems, volume letters A:, B:, and C: were reserved for two floppy drives (A: and B:) and the primary drive (usually a hard disk).