Just to follow up on my solution (stopped looking for how to make the external usb hard drive bootable!). EaseUS Todo Backup has a utility to create a bootable flash drive.(Only available in paid versions, not the free one.) So, just doing this and creating an image of my laptop hard drive makes a pretty simple emergency restore solution when I'm on the road. And the rest of the external hdd is available for backing up other files.
As a reference point, creating a full encrypted image of my internal drive with about 160GB of data took about 1.5 hrs via usb 2.0.