an information recording and reading apparatus for a recording medium , such as a magnetic disc or optical disc , which operates under the control of a central controller . the information recorded in the medium is stored in a memory and checked for errors during a time period subsequent to the recording operation by reading the recorded data in a read - after - write process . a comparator compares the content of the memory and the read - after - write signal to determine whether an error has occurred . if an error is detected , the data is re - recorded in a different part of the recording medium .