Optical discs are the main external backing store.
- Optical discs include compact discs (CDs) and Digital Versatile Discs (DVDs).
- They store digital data as pits on the surface of a reflective disc.
- Data is read by a moving laser beam across the surface of the disc
There are three types of CD
- CD-ROMs
Can only be used to read data ('Read-Only-Memory') - 650MB
- CD-Rs
Sold as blank CDs and can be written on once. After that they become CD-ROMs.
- CD-RWs
'Read and Write' - new data can be written on to the disc.
DVDs are like CDs, but hold more infomation/data - 8.54GB
CDs --> DVDs --> --> Blu-Rays In order of increasing capacity
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