I had this this problem.
My Sandisk stopped copying from my IPad (even though it said the SanDisk still had 400mb of space still left on it.)
I had 5193 photos and some videos on it.
So I transfered these photos from the SanDisk to another computer…
And deleted them from the SanDisk to make space.
Now it works again perfectly 
So the answer for me was: My SanDisk was full - so I cleared off some space on it.
