boot from external hdd
xp installed on my external hard drive (previously my internal hard drive of a laptop) doesnt boot on usb of other laptop
Is the version of XP on the hard drive you removed and are now using as an external drive an OEM version? If it is an OEM version the it probably will not work on another laptop. When booting the laptop have you selected the USB drive as the first boot device? I'm not entirely sure that what you are trying to do is possible.