What are primary and secondary storage devices?
Answer: A computer stores data using several different methods. Therefore, there are different levels of data storage, which may may be referred to as primary and secondary storage. A computer's internal hard drive is often considered a primary storage device, while external hard drives and other external media are considered secondary storage devices.
However, primary and secondary storage may also refer specifically to the components inside the computer. In this case, primary storage typically refers to random access memory [RAM], while secondary storage refers to the computer's internal hard drive.
RAM, commonly called "memory," is considered primary storage, since it stores data that is directly accessible by the computer's CPU. RAM is a high-speed storage medium that can be accessed with minimal delay. Because the RAM is connected directly to the CPU via the memory bus, the CPU can access data stored in RAM very quickly. For this reason, RAM is used to store data loaded by active programs and the operating system.
Hard drives are considered secondary storage since they are not connected directly to the CPU. Instead, hard drives send and receive data through an I/O bus, which may pass through a cache or other type of memory before getting to the CPU. Also, hard drives are not as fast a RAM, which means they cannot transfer data as quickly. However, unlike RAM, hard drives retain data when the computer is turned off. This is because hard drives store data magnetically, while RAM requires an electrical current.
While the computer's primary internal hard drive is the most common secondary storage device, other devices may be considered secondary storage devices as well. These include additional internal hard drives, external hard drives, and optical drives, which are used for CDs, DVDs, and other optical media.
Entered: August 7, 2009 – by Per Christensson
Category: Hardware
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Computers use a variety of data storage devices that are classified in two ways: one is whether they retain the data if there is no electricity and the other is how close they are to the processor [CPU]. Both types of storage are needed in all computers. In a personal computer, memory does not retain data when the electricity is off, but while it is on, it enables quick access to open files. A storage drive, however, allows you to permanently store data, so it's available each time you turn on the computer.
Volatile and non-volatile storage
The first classification of computer data storage is between volatile and non-volatile storage. An example of volatile storage is memory [RAM] that stores data only until there is no electricity powering the device. RAM allows your computer to have multiple files open and access any of them instantly. Some other examples of volatile storage devices are calculators.
Non-volatile storage is storage that maintains the data even when there is no electricity powering the device. An example is a hard disk drive [HDD] or solid state drive [SSD] that holds all of the data saved to your computer. There is other non-volatile storage, such as DVDs or flash drives. For more information about the difference between memory and storage, read here.
Storage hierarchy
Computer data storage devices are also classified by their distance from the processor, or CPU. The closest storage is memory, or RAM. This is the only kind of data storage that directly accesses the CPU. Memory includes processor registers and the processor cache, but these are included on the memory module.
Memory is volatile storage, so any information that goes into memory needs to be written to the main storage device to be retained permanently. Because data flows from memory to a storage device, it's considered secondary storage.
For most personal computers, secondary storage is the main data storage device. A hard disk drive or solid state drive holds all of the data; files, photos, programs, music, and movies, that the user wants to keep. Removable, external media storage devices such as flash drives and read/writeable CDs and DVDs are also secondary storage. A computer can't function without a storage drive, however. The storage drive also holds all the information the computer needs to run.
Tertiary storage is computer data storage that uses removable media, such as a tape drive, and it uses a robot to retrieve the data. This is rarely used in personal applications.
Conclusion
In common use, the hard disk drive or solid state drive is usually referred to as the storage drive. Because memory is volatile, it's hard to think of it as a storage device. And because personal computers rarely use tertiary storage, the storage drive is the main, and frequently, only non-volatile data storage device on the computer. Learn more about the difference between hard drives and solid state drives.