A cartridge for a magnetic disk drive has a rigid shell and a flexible door which covers an opening through which read/write heads engage the recording medium. The flexible door is guided outside of the shell to an open position as the cartridge is inserted into the drive. A projection on a flexible arm in the drive catches a hole in the flexible door to open it as the cartridge is inserted into the drive. The door which opens along the outside of the cartridge conserves space so that the recording medium can be large for a given form factor drive.