To make the secret message I originally explored a few ideas:
1. Copy a text and or image file to the disk
2. Find a copy of some floppy disk linux distro and setup an init script to display a message on boot
3. Write to the boot sector of the disk and hope someone is silly enough to put the disk in before turning on their computer
Number 1 would have been relatively easy, but not nearly as fun. 2 seemed doable however I had some difficulty finding a working distro and 3 sounded like oh so much fun.
So, the basic plan was to find some way to make a custom 'boot disk' that when the computer is powered on with the disk in the drive a secret message will appear on the screen. For those of you too impatient to read the instructable, you can find the code to make your own secret message floppy disks here:http://github.com/braingram/floppysecret