In optical or magnetic data storage, a writing technique in which each bit of data is read immediately after it is written. This enables an erroneous sector to be recognized before the next sector starts to be written and errors can be managed accordingly, generally by flagging the defective sector or block and repeating the same data in the next sector. Nearly all magnetic tape drives, and many optical disk drives, use this technique. See also DRDW.

The term is sometimes erroneously used in an optical-storage context simply to imply that written information is immediately ready for reading, without an intermediate processing operation such as would be required for photographic recording.