Reboot

The fastest way to do it :

# mkdosfs -C floppydisk.img 1440
# mount -o loop floppydisk.img /media/cdrom
# cp /path/to/files/* /media/cdrom/
# umount /media/cdrom

note: assuming /media/cdrom path is exists (most likely so), and you have root access.

credit: http://untitledfinale.wordpress.com/2007/10/09/create-mount-and-copy-floppy-disks-images-under-linux/