Windows 7 on a usb hard disk drive ?

I´ve got a Problem:
I want to install windows 7 on a external hard disk, but when I want to choose it in installations menu, there appears an error message, that it in not possible to install windows 7 on a externat usb medium.
I´ve got a notebook, so I can´t integrate the external hard disk in my Computer.
I´ve got windows 7 (32 bit) on my Dektop, and because it works so great, I want to install the 64-bit version on my Notebook, too.

Do you have an idea, to resolve the problem ?

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