Which kind of floppy drive do you have - USB drive or a intern drive with floppy data cable? If intern drive,does BIOS recognize and/or support floppy disk and what kind of floppy disk? While booting do you see light of the drive-LED?

If you have very old floppy disks the preformat may be damaged; therefore Knoppix cannot mount the disk. Try to new format a floppy disk.