Abstract: A disc storage file includes a multiplicity of closely spaced continuously rotating flexible or floppy discs associated with an accessing assembly for spreading the discs apart at randomly selected positions to accommodate a magnetic head. Apparatus for locating the disc interface (gap) to be accessed includes a pair of offset photocells which are light coupled to the disc gaps. The light applied to the photocells is guided in offset channels of a shroud and shaped congruent to the photocell surfaces by offset fiber optic shaping arrays. The offset arrays and associated electronics allow for accurate counting of gaps in the presence of fluctuating disc motion in the axial direction.