Instructions
Remove all memory cards from your GameCube.
Insert the non-working memory card into Slot 1 of your GameCube. This is the slot located above the port for the Player 1 controller.
Turn on your GameCube. On the main menu, select "Memory Card Screen." Your corrupted or wrongly formatted memory card will prompt the GameCube to display a screen asking whether you want to reformat the memory card.
Select "Yes" by highlighting the "Yes" button with your controller's D-pad and pressing the "A" button. You will be asked to confirm this action.
Select "Yes" once again and press "A" again. Your memory card will be fixed and ready for use.