the reason to format the card in the camera you use it in is not to erase the images. WHEN YOU FORMAT THE CARD IN THE CAMERA YOU USE IT IN, IT MAKES A NEW ALLOCATION TABLE. that table is why you could have the images on the card but be unable to get to or download the images.
this is why formating the card in the computer will not work. the computer does make the same allocation table that the camera does. also, you should not format the card in one camera then use it in another, the allocation tables will not be the same.
yes, you can get away with deletion only or doing what the opposite of what was mentioned above. but, the day you go on your once in a lifetime trip to hawaii is when you find out that it did not work. the only way then to fix the problem is to go back to hawaii and spend the first $10000 again taking another set of the same pictures.
it is simply safer and easier to do the formatting in the camera that you are going to use it in, and do it every time.
for the safety of the images on a hawaii trip and the $10000 that goes with, are you going to taken the chance? how many $10000 do you have and can afford to spend? having been to a great many national parks, there is no way that i will take the chance. not to mention about shooting a wedding for a fee, are you going to tell the b&g that they have no images the next day because you did not format and took a shortcut?
