The wiki says that we can't use SD with the Ethernet without disabling one and enabling the second
i.e: https://github.com/sirleech/Webduino/wiki/Webduino-webserver-hanging-after-a-short-period---fixed-by-disabling-the-SD-card

Another question at Stack Exchange and the answer was nearly the same
http://electronics.stackexchange.com/questions/67212/how-to-avoid-sd-card-and-w1500-spi-mixup-on-the-ethernet-shield

but I found the following article:
http://startingelectronics.com/tutorials/arduino/ethernet-shield-web-server-tutorial/SD-card-web-server/
When I follow I was able to use the SD & Ethernet together without any restrictions.

My problem now that I can't port the code from the provided link to use Webduino.