Optical storage and tape storage are useful for storing backup information.
CD, DVD and Blu-ray discs are forms of optical storage. They are a useful lightweight media for transporting data and programs.
Data is written onto the disc by a laser and is read from the disc by another laser. The discs are cheap, but they are vulnerable to scratches. They cannot store as much data as a hard drive. A CD stores up to 800 MB, a DVD holds up to 4.7 GB and a Blu-ray holds up to 50 GB.
You need to have a CD, DVD or Blu-ray writer on your computer to write information to the discs. CD, DVD and Blu-ray discs are useful for creating film or music discs which are compatible with hardware such as DVD players.
Big businesses and science projects, which process thousands of bytes of data every second, often use tape storage to back up their data. It is a very cheap and reliable way to store large amounts of data.
Memory can be direct or sequential.
Direct is memory also known as random access. This means that any location in storage can be jumped to at any moment, eg on a direct device such as a DVD, you can jump straight to a chapter.
On a sequential device, records are stored one by one in an ordered sequence, eg on a film reel or video tape you have to wind through to find the point you want - you cannot jump straight to it.
Magnetic tape storage is sequential which is slower than random access storage.
The speed of a storage device is measured in megabytes per second (MBps).