My sd card is a 16 GB and I've taken pictures just fine and have transfered them to my usb through laptop (hp 2000) without a problem. A few days later while doing this my laptop starts to show that some are not viewable. Most show but others don't. The ones that can be seen are type file JPG and the ones that can't be seen are NEF file type. Is this the reason why and if not how can i fix it? Please help.