Problems with nand write

Hello,

I want to rewrite the nand of the Mini6410 board.
But I can't access the SuperBoot.
I've flashed the sdcard, I can boot the Linux rootfs, but I can't access
the superboot to rewrite the u-boot, kernel and rootfs to the nand!
The serial terminal is just black, nothing appears.
What could be? What can I do?

I've already fused the sdcard with the superboot.