;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