This article shows how to resolve problems that occur when using Ethernet Shield and SD Card in Arduino. Author says below
Problems such as intermittent hanging of an Arduino sketch can occur when using the Arduino Ethernet shield with an uninitialized SD card plugged into the SD card socket of the shield. It is always best to disable the SD card before initializing the Ethernet chip, or disable the Ethernet chip before initializing the SD card.
For more details, please refer to the following link.
Tags : 201506, Arduino UNO, Ethernet Shield
Author : http://startingelectronics.org/, Not yet