Can you post a link to the SD shield/hardware?
I use an ethernet shield with a uSD card onboard, and it is quite possible to unintentionally "write" to the SD if you do not disable the SD SPI interface when setting up the w5100. Is there another SPI device on the SPI bus?It can also garbage up the SPI bus like you are seeing by using the SD card without disabling the w5100.
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