Abstract: An initialized (formatted) 1.44 MB floppy disk which is ready for receiving and storing files under the MACINTOSH.TM. operating system and the DOS operating system. Alternatively, the invention may be applied to any computer storage medium which may be written to and read by the MACINTOSH.TM. and DOS operating systems as if it were a disk. The storage medium is not bootable for either operating system. It contains a specially configured DOS BIOS parameter block in the first physical block, a DOS file allocation table in the second physical block, a MACINTOSH.TM. master directory block in the third physical block, and, by containing only zeros, an empty DOS Root Directory and an empty MACINTOSH.TM. Volume Bitmap in the fourth physical block.