tom tom 500 card problem

i have just bought a second hand tomtom500 and i wanted to add other stuff onto it so i was told to copy my card into a folder and format the card which i did and then copy the new stuff from the cd onto the card that was fine untill i put the card into the tt 500 a picture of the card came up with a big X across it ,so i formatted the card again and put the backup one back on but its still comes with the X and i cant get into my tt but if i open the card on my computer everything is on the card can anybody help please