my NBN modem is Optus supplied Sagemcom F@st 3864 router. just upgrade system to windows 10.
Internet and Wireless are all working fine.
In the menu options it appears that I can connect a USB external drive/flash drive and make it available across the network,
both as a file share and DLNA.
When I plug in the drive I can see that it is recognised (in the Router interface) but I can not find it on the network. it used to work well under windows 7.
You're confusing DLNA with File sharing, they are two different things.
To access the USB drive over the network, generally you need to tell your computer to look at the IP address of the router. By pressing Windows + R you should bring up the "Run box", in this type \\192.168.1.1 if that is the IP of your router, and in there you should find the USB drive you've plugged in.