You can refer to my post here first.
What you can do on AGX is just “mount file system” from sd. Not able to boot from it. (for sd only. USB and NVMe could be totally booted from)
And no matter what case, you need to flash the emmc before letting external device work. And flash.sh can only flash emmc for AGX. You need to prepare the file system on the sdcard in other machine (e.g. x86 host) before you plug it into AGX.
But flash step won’t need to be repeated again if you just switch between same interface. For example, from Sdcard A to Sdcard B.