A direct access storage device that is written and read by light. The most common optical discs in use are CDs and DVDs. As removable media, optical discs have advantages over the older removable magnetic disk cartridges. They weigh less, have higher capacities and are not subject to head crashes or corruption from stray magnetic fields. They also have a 30-year life and are less vulnerable to extremes of hot and cold. However, flash drives and external hard disks that plug into the USB port have given optical discs a run for their money as a transportable storage medium.