Possible corrupt card

I use a d700 and a new sandisk extreme. I have down loaded the files from my camera to my computer and have gone to convert the images in Adobe Free Dng Convertor as i always do. Only a few images have converted and the rest are coming up with an error message ‘there was an error writing the file’.

These images are very important. This has never happened to me before. I can view the images on the camera.

Any advice pleas