Your comment "I currently have to delete the file then save it so long as I save it within a folder on the drive" could be a clue. Are you trying to save the file in the root directory of the external drive? If so this could be a problem as there will be a limit to the number of files that can be stored in the root directory.
If you make a new folder (sub-directory) for the files you wish to transfer this could solve your problem. You may need to remove (delete) a file to make room for the new folder.