I just successfully copied a mp3 from my PC to the SD card through the card reader, so I don't think it's the card...just that particular movie. However-scratch all of that. I just tried the same movie again while I was typing this message and it worked fine.
Maybe I did accidentally lock the card with that little switch on the side. Who knows? But thank you all for your responses. Everything helped. I just love this site. Sometimes I have to remember that there are other places to surf!!!