So, this link shows wonderfully how to root into a USB: Boot from external USB stick / drive?
What I want to do, though, is root from the SD card by default, and then the first available USB if one is inserted.
In the end I would like to have a set up where my pi boots into command line from the SD root if nothing else is there, and it does not ever mistake one of my storage drives for a root source one.
I'm kind of going for a classic cartridge console sort of feel, but I also want it to be standalone because otherwise it's useless without the proper USB stick.
Can you change which root the boot process looks for on run time using conditional statements? What is the "hello world" of boot without root coding?
EDIT: Ideally I would like to be able to choose from a set of cmdline.txt presets based off what exists in the USB slots.