Can´t move files from atv to usb drive

Hi all. I´m using Transmit/Cyberduck to move content from Mac to ATV internal or external Drive (USB) Now i found out, that it´s not possible to move files from the internal ATVdrive to the external USBdrive. In both, Cyberduck and Transmit there is an error message. What is the reason for that and is there any other way to move file between the drives?