1) Yes I say that because I don't think that grub can support boot from USB external drive... but if you find a bootloader which can do that... I'm wrong

2) When your computer boots, it loads bootloader installed in MBR (Master Boot Record)... Then you choose your favorite OS (linux of course

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your bootloader knows that linux kernel to boot is installed in /boot directory (in fact /dev/hda1 for example)... So the kernel is booted and once the kernel is booted, it has to mount root filesystem (and then other like /home, /var...). So if you have compiled your kernel with USB mass storage support inside, you can tell him that rootfs is your USB drive (you tell him that with the option root=/dev/something)
3) according my answer, yes but it won't really be a boot disk, it will be a root filesystem disk... unless I am completely wrong at 1) and so it will be a boot disk
