Abstract: Apparatus for bit-by-bit duplication of data stored on a flexible diskette. A master disk is driven by a conventional DC drive, which provides a feedback control voltage responsive to the instantaneous speed of the master disk. A slave disk is driven from external index synchronization circuitry, which receives signals from the respective disk drives indicative of the angular position of the diskettes, and additionally receives the master drive control voltage. The index synchronization circuitry outputs a motor error correction voltage to the slave disk drive, thereby coordinating the rotation of the respective diskettes. Bits are copied only when the slave disk drive parameters fall within a narrowly defined window.