diff --git a/libraries/SD/examples/Files/Files.pde b/libraries/SD/examples/Files/Files.pde
index b306793d4..ca72e2193 100644
--- a/libraries/SD/examples/Files/Files.pde
+++ b/libraries/SD/examples/Files/Files.pde
@@ -6,7 +6,10 @@ void setup()
 {
   Serial.begin(9600);
   Serial.print("Initializing SD card...");
-  // On the Ethernet Shield, CS is pin 4.
+  // On the Ethernet Shield, CS is pin 4. Note that even if it's not
+  // used as the CS pin, the hardware SS pin (10 on most Arduino boards,
+  // 53 on the Mega) must be left as an output or the SD library
+  // functions will not work.
   if (!SD.begin(4)) {
     Serial.println("failed!");
     return;
diff --git a/libraries/SD/examples/ReadWrite/ReadWrite.pde b/libraries/SD/examples/ReadWrite/ReadWrite.pde
index 1267dce00..3dc4cf19c 100644
--- a/libraries/SD/examples/ReadWrite/ReadWrite.pde
+++ b/libraries/SD/examples/ReadWrite/ReadWrite.pde
@@ -6,7 +6,10 @@ void setup()
 {
   Serial.begin(9600);
   Serial.print("Initializing SD card...");
-  // On the Ethernet Shield, CS is pin 4.
+  // On the Ethernet Shield, CS is pin 4. Note that even if it's not
+  // used as the CS pin, the hardware SS pin (10 on most Arduino boards,
+  // 53 on the Mega) must be left as an output or the SD library
+  // functions will not work.
   if (!SD.begin(4)) {
     Serial.println("failed!");
     return;