A direct access storage device that is written and read by light. The most common types are CD, DVD and Blu-ray. As removable media, optical discs superseded the earlier magnetic disk cartridges because they weigh less, have higher capacities and are not subject to head crashes or corruption from stray magnetic fields. Optical media also have a 30-year life and are less vulnerable to extreme hot or cold.
However, as a transportable storage medium, they have mostly been superseded by USB-based flash drives and external hard drives. Following are the major categories of optical discs in order of writability.
Read-Only (Factory Pressed)
Read-only discs are pressed from a master at the time of manufacture and cannot be erased. They include the music CD, CD-ROM, DVD-ROM, DVD-Video and BD-ROM (Blu-ray). See CD
Write-once discs are recorded in the user's environment but cannot be erased. They include CD-R, DVD-R, DVD+R, BD-R (Blu-ray) and WORM discs, as well as magneto-optic (MO) and ultra density optical (UDO) discs in WORM mode. See magneto-optic disk
Rewritable (Phase Change and Magneto Optic)
Rewritable discs can be written and re-written numerous times. Employing phase change technology, consumer-oriented products include CD-RW, DVD-RAM, DVD-RW, DVD+RW and BD-RE (Blu-ray) as well as ultra density optical (UDO). See phase change disc
Magneto-optic (MO) disks combine optical and magnetic technologies (see magneto-optic disk
). See ISO/IEC 13346
, multilevel optical disc
, holographic storage
and legality of optical storage
Writability Optical Disc Types
Read only CD, CD-ROM
Write once CR-R