A method for modifying data read from read-only media during playback time comprises logically merging the on-disc directory tree and an associated off-disc directory tree. A logical directory tree is constructed from the data retrieved from the read-only medium, wherein the structure of the logical directory tree is identical with the structure of the directory tree of the medium. The method allows replacing content on the disc through downloaded content, e.g. replacing an out-dated trailer stored on the disc through a downloaded trailer for a new movie. The method further allows complementing or upgrading content on the disc, e.g. by downloading a new subtitle track from the internet.