a writing method suitable for practice with an apparatus having a rotary head assembly for the storage and retrieval of digital data on and from magnetic tape in cassette form or the like . the rotary head assembly is of the type having a rotary drum to which there are mounted a pair of magnetic read / write heads and a pair of magnetic monitor heads . the read / write heads alternately and overlappingly scan the magnetic tape for creating a series of slanting data tracks thereon . the monitor heads are spaced from the read / write heads not only circumferentially but also axially of the rotary drum for monitoring the data tracks being formed by the read / write heads after they have been overlapped by the subsequently formed tracks . the writing method is of particular utility in writing a block of user data over any selected target group of existing data tracks that have been written on the magnetic tape by another similar magnetic tape apparatus in which the axial distances between the read / write heads and the monitor heads may differ from those of the present apparatus . the block of user data is written over all but the first and last of the target group of existing data tracks , and dummy data is written over the first and the last .