Secondary Storage Devices:
The secondary storage devices are used to store data that is of larger size which can be accessed later. Since the main memory is costly, the size is generally very limited in a computer. A portion of a secondary storage which can serve as an extension of the main memory and can perform its job is called as virtual memory.
Hard Disks:
Hard disk is a magnetic disk on which you can store computer data. The term hard is used to distinguish it from a soft, or floppy disk. Hard disks can store more data and are faster than floppy disks. A hard disk, for example, can store anywhere from 10 megabytes to several gigabytes, whereas most floppies have a maximum storage capacity of 1.4 megabytes. In general, hard disks are less portable than floppies, although it is possible to port by removable hard disks. There are two types of removable hard disks: disk packs and removable cartridges.
Compact Disc (CD):
A CD or CD-ROM is made from 1.2 millimeters thick, polycarbonate plastic material. A thin layer of aluminum or gold is applied to the surface. CD data is represented as tiny indentations known as “pits” , encoded in a spiral track moulded into the top of the polycarbonate layer. The areas between pits dre known as “lands”. A motor within the CD player rotates the disk. The capacity of an ordinary CD-ROM is 700MB.
Digital Versatile Disc (DVD):
A DVD (Digital Versatile Disc or Digital Video Disc) is an optical disc capable of storing up to 4.7 GB of data, more than six times what a CD can hold. DVDs are often used to store movies at a better quality than with a VHS. DVDs can also have interactive menus and bus features such as deleted scenes and commentaries. Like CDs, DVDs are read with a laser.
The disc can have one or two sides, and one or two layers of data per side; the number of sides and layers determines how much it can hold. A 12 cm diameter disc with single sided, single layer has 4.7 GB capacity, whereas the single sided, double layer has 8.5 GB capacity.
The 8 cm DVD has 1.5 GB capacity. The capacity of a DVD-ROM can be visually determined by noting the number of data sides of the disc. Double-layered sides are usually gold-coloured, while single-layered sides are usually silvercoloured, like a CD.
Flash Memory Devices:
Flash memory is an electronic (solid-state) nonvolatile computer storage medium that can be electrically erased and reprogrammed. They are either EEPROM or EPROM. Examples for Flash memories are pendrives, memory cards etc.
Flash memories can be used in personal computers, Personal Digital Assistants (PDA), digital audio players, digital cameras and mobile phones. Flash memory offers fast access times. The time taken to read or write a character in memory is called access time. The capacity of the flash memories vary from 1 Gigabytes (GB) to 2 Terabytes (TB).
Blu-Ray Disc:
Blu-Ray Disc is a high-density optical disc format similar to DVD. Blu-ray is the type of disc used for PlayStation games and for playing highdefinition (HD) movies. In the past, there were other standards for such movies. A double-layer Blu-Ray disc can store up to 50GB (gigabytes) of data.
This is more than 5 times the capacity of a DVD, and above 70 times of a CD. The format was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amounts of data. DVD uses a red laser to read and write data. But, Blu-ray uses a blue-violet laser to write. Hence, it is called as Blu-Ray.
