A data storage system comprises a data storage device (2) that comprises a first file system (4) of a first type and a second file system (70, 72, 74, 76, 78) of a second, different type, and a file system control means (92) for controlling file system operations, wherein the file system control means (92) is configured to receive requests to write files to the first or second file systems, and to write to the second file system (70, 72, 74, 76, 78) in response to substantially all requests to write files that have at least one predetermined file name property to the first or second file system.