Magnetic disc cartridge and magnetic disc drive system

A magnetic disc cartridge comprising a large capacity magnetic disc medium held for rotation in a casing provided with an opening which gives a magnetic head access to the magnetic disc medium and is normally closed by a shutter, wherein the casing is provided with a first cutaway portion at a front corner between the front edge and one side edge of the casing, and a second cutaway portion which is adapted to engage with an engagement member of a smaller-disc drive system which is brought into engagement with the first cutaway portion, wherein the smaller-disc drive system exclusively receives a magnetic disc cartridge having a capacity smaller than the large capacity disc medium and wherein the second cutaway portion is formed on the one side edge of the casing and comprises an inclined surface inclined inward from a front end of the second cutaway portion and further comprises a shoulder portion which extends substantially normal to the side edge at a rear end of the second cutaway portion and a parallel portion substantially parallel to the side edge to form a substantially trapezoidal shape.