Ah ok, then you need it.
Have a look here:
--> Vom 3DS XL einen NAND Dump erstellen und zurück flashen
What you can do is solder the wires from the motherboard to ans USB connector (for example a micro USB connector) and then use the respective USB cable ans solder it to an SD adapter. Trying to use the USB cable to read and write via USB port won't work!