Nikon D70 memory card problems?

I'm having a problem with my memory cards. I have six of them. I use them to take photos then I put them on my computer and put the card back in the camera it won't let me take photos. Someone suggested reformatting the cards with my computer and I tried that but still none of them will work. What is it? What can I do?

Your first problem is who on earth told you to format in your computer? That somebody hasn't a clue and I sure wouldn't listen to anything about photography any more.

You always format in the camera, no where else. You never switch cards from one camera to another, never. You always format a card after every upload, always.

Now, try formatting one of these cards in the camera and see if it works.

When you removed the cards from the computer did you eject them, to safely remove the card from card reader? If you didn't put the card back into the computer and do so.

Reformat the card in the camera and see if that helps.

You should always reformat the card in the camera when you return it after you copy the pictures to your computer. Don't bother erasing the pictures from from the card (you can, but it doesn't make a difference).