I find if I format the card in the PC, it doesn't work in the camera.
If I format it in the camera no probs.
I also think that if you open and resave a pic from the PC to the smartmedia, you can't reuse the card in the camera until you reformat it.
What make of camera is it?
Note that some cameras (my Fuji 5700 for example) doesn't allow you to format the card in the camera if you've formatted it on the PC in the wrong format.