[U-Boot] NAND_ECC_NONE in u-boot question

Hi,

I am using 9G20 and set eccmode NAND_ECC_NONE in u-boot, but when I try to write some data from memory into flash with u-boot interface, it failed. (sam-ba can not read this page back from nand flash, it says a bad block). but I use NAND_ECC_SOFT, it is successful. why? anyone can give some help? Thanks in advance.