hello kevin.choi,
you should confirm your host machine has already update $OUT/Linux_for_Tegra/bootloader/system.img with your customized image.
please also check Flash Script Usage session, you may enable -G options to saves the image to the specified file, if you would like to clone your target’s image.
above -G commands only works with Nano-emmc, we don’t support cloning on Nano-SD.
BTW,
please also check similar discussion threads for reference, for example, Topic 127193, and Topic 141950.
thanks