Information on a spinning magneto-optic disk is overwritten by illuminating a spot on a track on the disk with an erasing beam in the presence of a biasing magnetic field, and illuminating another spot on the same track with a writing beam during the same revolution of the disk. The erasing beam and writing beam can be focused through the same lens. The erasing beam erases all bits to the same known value, so it is not necessary to control the writing beam according to the old values of the bits. Benefits include simpler control logic and less stringent positioning requirements than in the past.