Data storage devices are mainly classified into three types - primary data storage devices, secondary storage devices, and tertiary storage devices. This classification is made on the basis of the particular task performed by these different devices.
The amount of data stored on primary storage devices is less, and this memory is volatile in nature, while secondary storage devices have larger storage space, and this type of memory is non-volatile in nature.
Primary Computer Storage
Primary storage devices for computers are actually a part of the computer's main memory. They are the Random Access Memory (RAM) and Read Only Memory (ROM).
Read Only Memory
ROM is used to store data permanently. Data is typically stored on it at the time of manufacture or later by the user. The main disadvantage of this type of memory is that the data stored on ROM cannot be changed. The data access rate of ROM is much lesser as compared to that of the RAM. On the positive side of things, a ROM costs much less compared to RAM.
Secondary Computer Storage
Most storage devices that we are acquainted with come under this category. These devices are typically used to increase the storage capacity of the computer. Like the ROM, the secondary computer storage is also a non-volatile type of memory. The information stored on these devices is retained, even if the computer is switched off.
However, with time, they evolved to be a part of the central processing unit of the computer. Presently there are external hard drives available as well, which connect to the computers via USB or similar interfaces.
Data is stored optically on the CDs, and prevails for a long time. They are mostly used to back up data, and also for distributing copies of software programs.
Like the CDs, DVDs are also used as backup devices and also for distributing high quality video files and movies, and large software and games.
These storage devices are similar to the read only memory, with the main difference that data from a flash memory can be erased in a matter of few seconds, which is not possible in a ROM. Also with improvements in semiconductor technology, flash memories having greater storage capacities and higher transfer-rates are being developed.
USB Flash Drive
A USB flash drives comprises a flash memory that is accessible via an integrated Universal Serial Bus (USB) interface. It is a non-volatile form of storage, which allows data to be rewritten several times on it.
Most flash drives are very small in size, but have very large storage capacities. Modern USB drives can typically handle up to 1 million erase and write cycles, and can also retain data for as long as 10 years.