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 (L_DT) 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 (D_DT) 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.