[Soli]

But wouldnt it just skip the external usb drive if the external drive isnt bootable? That whats normal pc do.

depends on if the external drive has any partitions marked as bootable (regardless if a valid bootloader is installed).

I'm not particularly invested either way, but it strikes me as logical that if you want the box to boot from something other than the default (internal ssd), then you need to manually take steps for it to do so. And since you already need to connect the USB stick/HDD that you want to boot, I don't see the hassle of selecting it from the boot menu after powering on

Yes, that what I thought. If the MBR doesnt say it's bootable, then the BIOS will simply skip it.
But I disagree that the internal ssd should be prioritized. If I insert a bootable usb stick into my chromebox, then I obviously want to boot from it. I don't want to connect a keyboard and hit ESC, downarrow, enter. I think all of us that are even flashing your bios are well enough versed with computers that we dont "accidentally" use a bootable MBR on our external drives. I normally dont have a keyboard connected (except I do, but the Logitech K40x doesnt at that early stage). This would make it more intuitive to boot a live Linux distro, or OE from a USB stick without the hassle of connectiong a corded keyboard.

Anyways, my suggestion would that you include an option in your script to flash an alternative bios that automatically boots from USB. That would be great for those of us who like living on the edge..