Wayne,
Sorry for the confusion. Based on your description, If I understand you the Linux box is your client (not a samba server) and the WD N750 router is acting as a NAS (Network Sttached Storage) device, by virtue of the connected usb drives.
First, if the above is correct than your Linux box does not need to be configured as a Samba server. Samba installs with (amoung other more things) to basic features a samba-server and a samba-client. Since you are not maintaining files and folders on your Linux box that thw Windows machine needs to access then you do not need the server portion of samba running and configured in you Linux box. The purpose of the smb.conf file is to configure the Linux box as a server. Since you are not serving files from this compture the smb.conf is doing nothing. It has no bearing or ability to configure the router.
Every major Linux distribution's file management application (for Ubuntu pre 13.04) This is Nautilus and Network Manager comes with Samba-client functionality builtin into their applications. Because of this Nautilus can see Windows (and other) Shares residing on computers it can see on your network.
What this all means is that your router is controlling the share accessibility not the Linux box. The accessiblity to shares on the router must be set up there not with Samba.
If you can see the external drive shares from your Linux boxes file manager (Nautilus or other) and you are prompted for a password then Based on the WD N750 manual if you setup usernane and password for your external storage shared access (on the router), you would enter this when prompted. If you did not set up userame and password than the manual says to enter "guest".
I hope this is helpful. Let me know if it doesn't work or there are more questions.