Question : How to boot from external USB HDD

I would like to be able to boot  from a USB external hard drive running Vista service pack two.

I have installed in an external USB 2.0  enclosure a hard drive that was in my notebook computer that was running Vista service pack two. When I connected this external hard drive to my notebook and tried to boot from it using my BIOS capable boot from USB hard drive  capability I was unable to boot to this external HDD.

My current operating system on the notebook is Windows 7 ultimate version.

Could someone please tell me what I need to do in order to get the external hard drive to boot into the Vista operating system.

Thanks in advance

Answer : How to boot from external USB HDD

These two prerequistes must be for filled

1. BIOS is set to boot from a USB, this is typical on most modern mainboards. Most people get this step ok...

2. Your USB must be formatted to be bootable which is not default by most USB manufactures.
The webpage below gives instructions on how to make a bootable USB with command prompt, setting the parition as an active bootable partition
http://www.maximumpc.com/article/howtos/how_to_install_windows_7_beta_a_usb_key