Write pass identifier

A method of encoding data with a means to distinguish between new data and incompletely erased old data when restoring said data from a magnetic tape storage device comprises: arranging a byte stream of user data into a plurality of track blocks; adding to each track block a track block header containing a write pass identifier; as said data is written to tape, the write pass identifier of the data already on tape is read and the Write pass identifier of the data being written to tape is incremented.