Installing XP to a USB-Bootable Device

I read explicitly many times that it was forbidden (technically) to install XP on an external drive ; Microsoft will forbid this beacuse it would allow copying very easily; however , what is, to my knowlegdge, possible , is to MAKE a copy of your hardrive or of the active partition on an external drive , which would let you to recover exactly the drive in the state where it was before the crash , or at the moment where the exact image was done; recommandable softwares are in that respect : DRIVE IMAGE (powerquest now absorbed to my great disappointment by Symantec whose tech service is undescriptibly awfull) or Paragon exact backup ; while the technology of drive ima