A method of appending data to compressed data stored on tape (10) in the form of records (CRn) wherein compressed data is stored in groups (Gn) independently of the record structure of the data and each group has an associated data structure (4), containing information relating to the group contents in terms of entities (En), where an entity can contain more than one record, and means (H) for storing information on the number of records in each entity (En) characterised by locating the entity containing the last record to be retained and changing said stored information to indicate the number of wanted records in that entity and writing the data being appended to a subsequent new entity. <IMAGE>
