Magnetic disk apparatus magnetic disk and disk cartridge

A magnetic disc apparatus using a magnetic disc adapted so that recording and/or reproduction are carried out by a magnetic head unit which undergoes floating force by air flow produced as the result of the fact that the magnetic disc is rotated serves to selectively carry out loading of a first disc cartridge within which there is accommodated a first magnetic disc adapted so that flat recording tracks are provided at a disc substrate formed by molding synthetic resin and a magnetic layer is provided in a manner to cover the recording tracks, and a second disc cartridge within which there is accommodated a second magnetic disc adapted so that information uneven pit trains are formed at a disc substrate formed by molding synthetic resin and a magnetic layer is formed in a manner to include bottom surfaces and upper surfaces of information uneven pit trains, the magnetic layer being magnetized in the state where magnetization direction of the magnetic layer portions on the bottom surfaces and that of the magnetic layer portions of the upper surfaces of information pit trains are different from each other so that information such as program, etc. are recorded. Thus, new program can be installed into computer only by exchange of the disc cartridge.