The problem is due to the .DS_Store files with Mac operating systems.
1. Edit the smb config file on the Readynas in /etc/samba/smb.conf
2. Remove the .DS_Store reference from the "veto files =" line
3. Reboot the NAS and the copy should now work.
Note: The Samba config can get re-written by the NAS at any point, so this is only a temporary fix or you might have to repeatedly apply the steps above.