Is the file system format of the external drive exactly the same as on the internal drive? MacOS Extended (Journaled) and not case sensitive, if the format of the internal drive is case insensitive? If the file system format is different, some files can be creating the error -36, because the file names may not work on the external drive.
You can check the file system format for each drive with "File > Get Info" if you select the icon of the drive in the Finder.