I know of no laws that stop you from copying the O\S and contents of your hard drive to a back-up disk.Would this make products like drive image illegal? Also what of raid boards that mirror the entire contents of one drive as security.
What does concern me is that you want to mirror it to an external hard drive, usb?
If your present system fails and you want to reinstall are you thinking of installing it from the external drive? This may have problems. It has been discussed in the past of booting from an external usb drive and I don't think that a solution has come up. Usually you have to have windows to load the usb drivers before the peripheral can work.
I would of thought that an internal IDE hard drive would be better suited. You could mirror your drive and store it safely away.
If you were thinking of something other than a USB external drive or think this is a load of tosh.
Then disregard. :)