RAID – Redundant Array of Inexpensive / Independent Disks

Protecting persistent stored – but dynamically changing – data is the most basic task for achieving some kind of business continuity.
In the early days of IT, when paper based media like punch cards or paper tapes were in use to operate an electronic machine. Output from a program is either printed on paper or stored on the same media as the input. The advantage of this technology is that the results of the computation resists most environmental hazards like mechanical force – drop from a table / hit by a hammer – or strong electromagnetic fields. Even creating exact copies of data with electro-mechanical devices like card puncher or teletype terminals is easy to achieve (or at least it was at the time those media where in common use).
The drawback of paper media is the very low density of information per physical volume of media as well as the inability to reuse / overwrite outdated information.
So pretty soon magnetic media was used to store data as well as code. Either written in a serial format – most popular magnetic tapes – or randomly accessible on a magnetic disk.
But the challenge of magnetic media is the sensitivity to environmental hazards- especially physical or electromagnetic forces. So creating copies of the data as part of the write process or immediately afterwards is good practise to avoid loss of information. For disks