A data backup system can be operable to determine whether to send a data file segment to backup storage. The data backup system can comprise a comparator operable to compare a boundary parameter of a candidate segment proposed for sending to backup storage to a boundary parameter of a previously stored segment to determine whether the candidate segment potentially corresponds to the previously stored segment. The data backup system can also comprise a comparator operable, if it is determined that the candidate segment potentially corresponds to the previously stored segment, to compare a checksum value for the candidate segment to a checksum value for the previously stored segment to determine whether the candidate segment has previously been stored to the second location. A storage engine can be provided to send to backup storage a segment determined not to have previously been stored. Thereby a single instance store can be implemented whereby a modified segment can be identified and a segment boundary adjusted to avoid re-storage of previously stored data.