I'm pretty sure that this uboot works for guruplug server plus, are the commands you're using the following?. A change in the addresses could make you plug get bricked
tftp 0x6400000 u-boot.3.4.27.bin
nand erase 0x0 0x100000
nand write.e 0x6400000 0x0 0x100000
Could you post the commands you are using?