A USB disk can be connected to Fritzbox as a network attached storage (NAS). In case one wants to connect this to ownCloud through the External Storage App of ownCloud, then this is the configuration that at least worked for me:
Host: the IP of the Fritzbox, e.g. 192.168.178.1
Password: the password for the fritzbox_username
Remote Subfolder: /some/folder/on_the_NAS
The brown square on the SMB configuration page of owncloud should then become a green circle.