If your floppy disk is damaged before,during,
or after you save a file to it your computer will
give you an error message like you described. When
you format a floppy you can see whether or not you have
damaged sectors on it by clicking on the Display summary
when finished radial. I've had new disks that were damaged
and still I was able to save files to the damaged sections
at least my system reported the files as being there, but
when I tried to copy them off of the floppy I would get
the error cannot create file or something similar. Try
using some disk utility to see if any sectors are bad.