Ethernet & SD - SD working once.

Using Arduino Programming Questions
6

You may find the some reset actions will cause either the w5100 or the sd card to fail the begin() function unless you disable one while setting up the other. I use something like this:

void setup() {
   Serial.begin(9600);

   // disable w5100 SPI while setting up SD
   pinMode(10,OUTPUT);
   digitalWrite(10,HIGH);

   Serial.print("Starting SD...");
   if(!SD.begin(4)) Serial.println("failed");
   else Serial.println("ok");
   // SD.begin() returns with its SPI disabled. Good SD library!

   Serial.print("Starting w5100...");

   if(!Ethernet.begin(mac)) Serial.println("failed");
   else Serial.println("ok");

   // Ethernet.begin() returns with the SPI enabled (bug?), so disable it. Bad Ethernet library!
   digitalWrite(10,HIGH);

   // rest of your setup
}