Cannot get my computer to boot from an external device

My laptop's hard drive crapped out this week, and to get it replaced I have to send it in and they'll send me a new one. But it's Christmas, plus there's shipping time so it will be a while. To tide me over I had an idea. I have the hard drive from my old desk top in an external case making it a usb drive (also plugs into the wall).

So I figured it would be easy booting from that. I changed the BIOS on my laptop to allow external booting, changed the boot order to look for external devices first and removed the broken drive. However, when I try to boot up I get a message that says "Operating System Not Found"

I have a few suspicions:

1. I looked online at how to boot from USB and most sites expect you to be booting from a USB drive or USB external hard drive fresh from the box. They tell you you need to format, partition, install stuff, etc. I figured since this is an internal hard drive that stuff would be unnecessary. It has windows and all the other good stuff to make a computer run. I may be wrong.

2. The hard drive enclosure seems to only want to turn on when it is plugged into a computer that is turned on. Which means that the computer might not be able to see it is there until after the boot sequence has ended.

3. Something more complicated that I do not know about.