Abstract: A sector formatting and record addressing scheme and associated electronic recording and detection control systems for a removable cartridge disc used in a magnetic information storage unit. Indelible sector mark and record address/format information are electronically pre-recorded on the disc at the factory using a high precision cartridge writer. This cartridge writer phase-locks the recording clock to a magnetically recorded clocking disc rotating on the same shaft as the disc via an associated position transducer to write an exact number of bytes on each track having a precise physical relationship to each other. Separate recording clocks having a wide bit-frequency spectral separation are used to write sector mark and address/format data to provide maximum sector detection reliability. The cartridge writer control system computes the optimum length of sector code for the customer-specified record size.