For many using a memory card is simply putting the card in your camera and then shooting away. If you are guilty of doing this, you'll want to read this post more thoroughly.
Anytime you put a card in your camera it needs to be reformatted. Why? Because in the process of reformatting the camera will add software to the disk making it possible for the camera to write on and read the disk. If there is any other software on the disk, it may conflict with the camera's software and you may experience a write failure.
If you remove a card from your camera and put it in your computer to download the images, you must also reformat the card when it is returned to your camera because the computer may have also added software to the card in order to read it.
If you start with a new card, you must reformat the card when you place it in your camera. Every memory card comes pre-installed with software. Again, by reformatting the card you allow your camera to add its software and to help it work seamlessly with your camera.
So the rule of thumb is anytime a card is introduced to your camera, be sure to reformat it in the camera.