Q: IPAD deleting more than just photos from SD card??

I used to import photos from SD cards in my game cams to my PC all the time, delete photos from SD card, recycle, with no problems. Now I have an IPAD. The first time I import with each SD card using CCK, it seems to import OK. I then delete the photos from the SD card using the feature on the IPAD, and put the same SD card in the camera, Later, when I retrieve the SD card I see on the camera that there are lots of photos on the card. But now when i try to import again on the IPAD, I get the error message 'CONTENTS NOT AVAILABLE. cannot read the connected storage media.' I put the card in the PC and it tells me the card needs to be formatted. Can anyone tell me what's going on here? I understand what is being said about the DCIM directory and filenames, but something is wiping all that out and I can't figure out what. Please help. THanks.