I found a set of tutorials including this one at http://startingelectronics.com/tutorials/arduino/ethernet-shield-web-server-tutorial/SD-card-web-server/
and loaded it straight into a UNO, where it worked fine with a genuine Arduino Ethernet shield. If I load exactly
the same code into a DUE, it does not work.
I tried adding some serial strings to help debug, and it looks like it runs the 'setup' routine fine, and starts the 'loop' routine ... but it never detects a client calling into it, so never replies.
Any ideas ?