A system for detecting the servo sectors of a rotating magnetic disk solely from read pulses representative of information read on the disk. Each servo sector has a first or intersector gap, an automatic gain control data zone, a second gap, and a track data zone. Sector pulses indicative of the servo sectors are generated when all these gaps and zones are detected. Preferably, for the production of a write enable signal, the sector pulses thus produced are further compared with window pulses obtained by imparting a prescribed delay to the sector pulses. Writing on the disk is enabled when the sector pulses agree in time with the window pulses.