Storage Definition

Storage unit facilitates the data storage in computer so that it can be kept safe for future use. There are two types of storage – primary or main storage and secondary or auxiliary storage. We generally call memory for primary storage and storage for secondary or auxiliary, though memory and storage words are used almost interchangeably.
Paper Tapes, magnetic tapes, floppy disk, hard disk, compact disks, Blu-ray disks etc. Are some examples of secondary storage devices.

Data storage Units
Computer storage is measured in bytes, kilobytes (KB), megabytes (MB), gigabytes (GB) and increasingly terabytes (TB). One byte is one character of information, and is comprised of eight bits (or eight digital 1's or 0's). Technically a kilobyte is 1024 bytes, a megabyte 1024 kilobytes, a gigabyte 1024 megabytes, and a terabyte 1024 gigabytes. This said, whilst this remains true when it comes to a computer's internal RAM and solid state storage devices (like USB memory sticks and flash memory cards), measures of hard disk capacity often take 1MB to be 1,000,000 bytes (not 1,024,768 bytes) and so on. This means that the storage capacity of two devices labelled as the same size can be different, and which remains an ongoing source of debate within the computer industry.

Storage devices

HARD DISK

Spinning hard disk (HD) drives are today the most common means of high capacity computer storage, with most desktop and laptop computers still relying on a spinning hard disk to store their operating system, applications programs and at least some user data. Traditional, spinning hard disk drives consist of one or more disk "platters" stacked one above the other, and coated in a magnetic media that is written to and read by the drive heads. As discussed in the hardware section, hard disk drives can transfer data directly to other computer hardware via a range of three interface types (SATA, IDE/UDMA, or SCSI) and come in a range of speeds from 4200 to 15000 revolutions per minute (RPM).

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COMACT DISK

Is a very mature, low-cost and reliable storage mediam particularly well suited for most personal computer users for incremental data archiving, as well as for the physical exchange of moderate-sized qualities of data. Writable compact disks can be either CD-R (which are a write-one media) or CD-RW (to which data can be written and erased typically a few hundred times). The storage capacity of a compact disk is up to about 700MB for CD-R and somewhat less for CD-RW media (and depending on the format used to write the data).

FLSH MEMORY CARDS

Flash memory cards were developed as a storage media for digital cameras and mobile computers. They consist of a small plastic package with a contact array that slots into a camera or other mobile computing device, or an appropriate memory card reader. Such readers usually have several slots (to accommodate the various formats of flash memory cards now available), and can either be integrated into a desktop computer or laptop's case, or connected via a USB port as an external hardware unit. In addition to still and video digital cameras, many mobile phones, tablets, netbooks, media players, audio recorders and televisions now also have slots for reading and writing a flash memory card.