Can't retrieve some photos from Panasonic Lumix...

Question

Photos from a number of sessions can't be read either by the camera or my card reader drive.The photos I could access I copied and deleted from the card. The 'lost' photos can still not be found, but the card is 80% full. This problem occurred before, but after removing and replacing the card (as suggested in the troubleshooting) numerous times, the photos returned.Card has been formatted by the camera.Card: SD ultra II 1GBBought the camera new 2 weeks ago.