Possible solution.
I had this same problem and I found that some of the files I was copying had become locked. Look at the icon of the files you're trying to move and see if any of them have a lock symbol on the file's icon (very small).
To unlock them, get info (command i) and untick the locked checkbox. This solved it for me. I didn't lock these files, they were downloaded from my camera, and for some reason a few of the images got locked during the download.
Hope this helps.
I had this same problem and I found that some of the files I was copying had become locked. Look at the icon of the files you're trying to move and see if any of them have a lock symbol on the file's icon (very small).
To unlock them, get info (command i) and untick the locked checkbox. This solved it for me. I didn't lock these files, they were downloaded from my camera, and for some reason a few of the images got locked during the download.
Hope this helps.