Sandisk memory card

Has this card EVER worked with your camera? Since you didn't mention the model number of the Canon, it's possible that you have a compatiblity issue here. Most 4GB SD cards these days are actually SDHC cards, which store memory differently (in sectors, like a hard disc drive, rather than bytes, like your main memory). All SDHC compatible cameras also support SD cards, but cameras sold before the SDHC standard was created will not support SDHC memory cards.

There's a chance that Canon has a firmware upgrade to solve this. You would have to look on the Canon USA web site for this, based on the model number of your camera.