You can also refer to this post I just wrote to another user few days ago to understand some concepts about those boot from external device.
We didn’t try to flash sdcard image to a usb drive before, so I cannot give you a solid answer. But if you want to remove the sdcard and only let usb to do the work, you need to make sure usb is able to load the extlinux.conf on the usb but not the sdmmc.