I'm using an Uno to record data to an SD card. The SD shield in use is the one available at Radio Shack.
It seemed to me that it would be appropriate to include some error indications, so I created some LED blink patterns that would be invoked if the SD card were missing or if a file descriptor was not returned. That is, if the begin() fails (indicating a missing SD card), a flag is set, and in the loop the main program is disabled and the card only blinks its LED to indicate the failure. The open() is programmed similarly.
The problem is that I discovered that the LED doesn't blink its error pattern if the Uno is powered up without an SD card.
After some testing, I discovered that both the begin() and the open() can cause the LED pattern not to blink.
The sketch works fine if there is an SD card in the slot.
So I'd suggest that error returns perhaps have not been fully programmed in the SD library. Is this library open source? If so, I'd be happy to fix it myself. If not, I would ask that the author fix it. Thank you.
(I make it a practice to have an LED blink pattern at power-up time, to identify which sketch is loaded. That also fails to work when the others do.)