Patrick Shanahan suggests (with undeserved politeness):
You have the directory /media/floppy open in konqueror or, perhaps, an xterm window, something is using /media/floppy. Close that app or leave /media/floppy directory and it should no longer be busy.
No, I'd thought of that before I posted. But just for good measure I closed down everything (even Mozilla, which I've now reopened for this message) other than the konsole, and tried again, this time using umount /media/floppy This gives me the same bull about the device being busy. As for an xterm window, my Linux book doesn't describe what it is in sufficient detail for me to understand. (Funny, I always thought my IQ was over 60 until I started to use Linux three weeks or so ago.)