I want to say thanks for the participation, and Aaron, I got your message to head on over here and your SD card mystery might actually have some common ground in solving in relation to my 3G SIM module add-on. Take a look at the regular LAN Turtle of mine (see URL link) and you will notice on 'page 4' that depicts the bottom side image of the LAN Turtle near the top-right corner is what appears to be a serial (UART) which has 4-copper pinouts. Granted, my observation from comparing images of other LAN Turtle 3G types shows a 3G SIM module affixed to its 'top side'. This makes me wonder if the UART pinouts traverse the board (i.e., has soldered points that protrude through the mainboard to those 4-pinout (UART) connections, OR I also noticed a 2-pinout copper connection points near the top right corner of the mainboard (e.g., top side) near the 'top-right' corner which could possibly be another serial connection point. Either way, as long as the mainboard architecture has NOT been modified between the different LAN Turtle variants then X-MIGHT-MEAN-THE-SPOT.
My intention is to diagnose/test both pinout connection on each side of the mainboard. In theory, the UART is capable of handling most serial communications which is optimal for both serial based connections, which of course, an SD card uses a similar serial-based protocol called, "SPI". Anyway, we should continue working together and comparing notes given our similar interest to modify the LAN Turtle.