1) I do not understand your answer....edit
Have you copied just the /boot folder onto usb stick and hope to embed grub or have you copied all folders?
If you test kernel you do not need other folders but boot will need populating and usb partition needs to be bootable and bios needs to have boot order changed to usb (on my system treated as a hard drive) before your normal hard drive
2) your normal system...must have grub files in /boot but what is your bootloader if no grub in mbr of Ubuntu?
3) you need to be careful specifying root (hd-devicenumber). Please read this....
if grub was on hard drive of mbr we could guide you better.
mbr jumps to grub menu
Press C for commands
root (hd....and press TAB key to allow grub display each drive, if usb has a bootable flag.
4) I am not sure how you boot if grub is not in mbr...so whats your bootloader pls?