Can't boot from usb-hdd

Hi,
this is my 1st time here hoping to find help.
Well,
In my pc I have installed (in one of the hdd's) windows 2008 Server.
I want to back it up in a 2TB external usb2 hdd and then to be able to boot from usb.
Steps I follow:
1) Clone partition with 2008 to usb drive. Now I have a new partition (let's say PartB)
2) Set partB active
3) Copy the bootsector from Windows 2008 dvd issuing the command: bootsect.exe /nt60 {driveletter} /force
4) Change bios settings de-activating all the hard disks for not booting from them and setting the boot priority from usb-hdd.

What else should I do because when I try to boot I have the error
DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER.

Thanks
Nikos