|"couldn't i just have it plugged in at boot, and then select that as the boot up device?"|
You can set the bios Boot Order or similar setting to boot the computer from the USB connected drive - it may already be set that way - but if it has an existing operating system installation of Windows 2000 or higher Windows operating system on it, all you'll get is a blue screen error message.
How I found out about that....
I have a hard drive that was originally in a laptop a friend owned. It has an XP Home installation on it, and it's an IDE drive.
I bought a larger IDE hard drive for her, installed it in an external enclosure, and used a program in Windows that was installed on the orginal drive to clone the contents of the entitre original drive to the larger drive, then I removed the larger drive and installed it in the laptop. The larger hard drive loaded the Windows installation fine
Later on, I installed the original drive which still had all of the same data on it in the same external drive enclosure and tried to boot the computer from it.
All I got was a blue screen error message.
I looked up my situation on the web and found that's exactly what happens when you try to load an existing Windows 2000 or higher Windows operating system installation from an external hard drive, due to Microsoft does not allow that.
The blue screen error message is generated by the Windows installation on the hard drive.
It's the same situation whether the external hard drive enclosure is connected to the computer via a USB, firewire, or eSATA connection, and whether the hard drive inside the enclosure is SCSI, IDE, or SATA.