A method for backing up deduplicated data from a deduplicated data source comprising a plurality of data blocks, the deduplicated data comprising: - for data blocks that are identical, a first copy of that data block with other copies replaced by a pointer to that first copy, wherein each of the pointers and the first copy being hereinafter called a reference to that data block, - and, for data blocks that are unique, those unique data blocks, wherein each such unique block being hereinafter called a single reference to that unique data block, the deduplicated data being provided in a data stream and the data blocks of the stream being arranged into plural virtual volumes; the method comprising: determining, for each volume, references to each of the data blocks in each volume; determining a percentage of a total number of references in the volume that are multiple references to one data block, comparing, for each volume, the determined percentage against a predetermined threshold percentage; for any volume for which the determined percentage exceeds the threshold percentage, backing up all data blocks of the volume on data storage media; and, for the remaining volumes, for which the determined percentage does not exceed the threshold percentage, determining the existence of multiple references to each of the data blocks in a data stream of data blocks received from the remaining volumes and backing up on data storage media a single copy of any data block for which only a single reference exists, and backing up (135,325) at least two copies of any data block determined to have multiple references in the data stream of data blocks received from the remaining volumes, each of the at least two copies being stored on separate data storage media. - for data blocks that are identical, a first copy of that data block with other copies replaced by a pointer to that first copy, wherein each of the pointers and the first copy being hereinafter called a reference to that data block, - and, for data blocks that are unique, those unique data blocks, wherein each such unique block being hereinafter called a single reference to that unique data block, determining, for each volume, references to each of the data blocks in each volume; determining a percentage of a total number of references in the volume that are multiple references to one data block, comparing, for each volume, the determined percentage against a predetermined threshold percentage; for any volume for which the determined percentage exceeds the threshold percentage, backing up all data blocks of the volume on data storage media; and, for the remaining volumes, for which the determined percentage does not exceed the threshold percentage, determining the existence of multiple references to each of the data blocks in a data stream of data blocks received from the remaining volumes and backing up on data storage media a single copy of any data block for which only a single reference exists, and backing up (135,325) at least two copies of any data block determined to have multiple references in the data stream of data blocks received from the remaining volumes, each of the at least two copies being stored on separate data storage media.