use usb as booting disk

;format usb disk into fat32:

mkfs.vfat (mkdosfs) -F 32 /dev/sdb1 -n usb

;be careful to backup your old MBR:

dd if=/dev/sda1 of=/dev/sdb1/MBR-backup bs=512 count=1

;write booting image file into usb to make a booting disk

dd if=/somewhere/somebootingimage.img of=/dev/sdb1