As always, please note that single NIC setups are not supported, for the reasons documented in the FAQ. For any new people reading this, you should avoid single NIC setups.
I'm assuming you've shut off DHCP, correct?
The "media radar" does not work on the external network. This is why LinuxMCE is not automatically detecting your storage.
You may want to consider reorganizing your network to follow the guidelines. LinuxMCE does a great job at providing a central point to access all your your storage devices. To do this, you put all your storage devices on the internal network, and access them via the core's share. Setting up a segmented network will allow you to maintain separation between your existing network and the LinuxMCE network. As you've already seen, it's fairly easy to configure the firewall to allow you to access the core's shares from the external network. It would look something like this:
Internet ---> (WAN) Router (LAN) <--- (External) Core (Internal) ---> Network Switch <--- Storage Devices
Then you can access the core's file share to access all your storage devices. New storage devices will be automatically detected and configured, eliminating the need to reconfigure your file clients.
If you've opened up the firewall, you can add storage devices manually, but this should be avoided. This approach is not flexible, and you will not get all the benefits of the media radar, automatic device detection and configuration, and such.
http://wiki.linuxmce.org/index.php/Network_Attached_Storage#Add_NAS_to_the_.22External.22_LAN
