If an error occurs in processing then the transaction and master files in which the error occurred can be discarded and the backups can be used to re-run the process. Copies of the master files should also be kept off-site. Three set of master files should be kept as a minimum. The current set. Yesterday's set in case there is an error in the current set.
All data is written to two disks simultaneously so both disks contain exactly the same information.
Preferably, the disks should be stored at different sites. Disaster Recovery Back-ups should handle most routine errors. If there is a fire, flood, storm, earthquake or other natural or man-made disaster which destroys the entire system then alternative arrangements are needed. Some organisations will be ruined by several hours of computer failure. Banks and other financial institutions are examples.