External storage

In computing, external storage comprises devices that temporarily store information for transporting from computer to computer. Such devices are not permanently fixed inside a computer.

Semiconductor memories are not sufficient to provide the whole storage capacity required in computers. The major limitation in using semiconductor memories is the cost per bit of the stored information. So to fulfill the large storage requirements of computers, magnetic disks and optical disks are generally used.

Optical media are the media that use laser light technology for data storage and retrieval.

Optical storage devices

CD is a book for Compact Disc. The speed is much lesser than a hard disk. The storage capacity is 700 MB. Types of CDs include:

DVD stands for Digital Versatile Disc. Its speed is much faster than CD but not as fast as hard disk. The standard DVD-5 technology has a storage capacity of 4.7 GB. The storage capacity changes with the standard used. Its storage capacity (6k GB) is much higher than a CD (700 MB). It is achieved by a number of design changes.

Blu-ray storage capacity is up to 50 gigabytes (or even 100 GB) of data. The Blu-ray Disc (BD) is a digital optical disc format. It was originally created to take the place of the DVD format due to its expanded storage capacity. The name "Blu-ray" is derived from the use of a blue laser that is used to read the disc. This would be in contrast to the red laser used to read DVD Discs.

M-Discs are available as DVD or Blu-ray discs. They are supposed to preserve data up to 1000 years.

Flash memory is a form of solid state memory. It was invented in the 1980s by the Toshiba company. Flash memory is a particular type of EEPROM (Electrically Erasable Programmable Read Only Memory). It is a non-volatile memory. It retains the stored information without requiring a power source. It is called solid state memory because it has no moving parts. Flash memory is different from the regular EEPROM. In case of EEPROM data are erased one byte at a time which makes it much slower. On the other hand, data stored in flash memory can be erased in blocks. That is why it gets its name “flash memory” because the chip is organized in such a way that a block of memory cells can be erased at a single time or “flash”.

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Wikipedia