Hi,
We have checked and confirmed the above method does not work for Jetson Nano with SD card slot. Please try this method:
How to clone Jetson Nano SD Card? - #19 by mdegans
You can mount the SD card to Ubuntu host and run the command like:
$ sudo ROOTFS_DIR=_PATH_TO_ROOTFS ./jetson-disk-image-creator.sh -o sd-blob.img -b jetson-nano -r 300
Please set ROOTFS_DIR to the mounted SD card.