Once you're done with formatting the floppy disk and you have a filesystem written on it, there is something else that must be done before you can write to it: mount it
su --press Enter, enter root password, then press Enter again
mount /dev/fd0 /mnt/floppy
Now you can write to your diskette by using
cp somefile /mnt/floppy
ls -al /mnt/floppy
instead of copying to /dev/fd0. When you're done with it, unmount the floppy with
And don't forget to also 'exit' the root account, which is use for all the steps above. If you want to be able to write to your diskette as a regular user, look into the 'man mount' documentation for options that can be passed to the mount command (such as umask).