Bootcamp Assistant thinks SSD I'm booting from is an External Drive — help?
Apple does not support running Windows from USB drives. This does not mean you can not, just that Apple does not provide a way to do so. This also does not mean you can run Windows from an USB drive. I lot depends on which Mac you own. For sure, the Boot Camp Assistant is not going to help install Windows on a USB drive.

After installing Windows (internally) using the aid of the Boot Camp Assistant application, Windows boots and executes exactly the same as if you installed on a PC. In fact, you can install Windows on your Mac without OS X installed. One user posted where the Boot Camp Assistant was used to create the USB Windows Installer. Once booted from this installer, the internal disk was wiped clean of OS X and Windows was installed as the only operating system.

The company twocanoes offer a product called WinClone. They claim it can be used to migrate Windows to an USB drive and then booted using a Mac. See the link Migrate Windows 8 or higher to an External Drive . The key word here is migration. Windows was installed somewhere else, then migrated (copied) to the USB drive.

Instructions for installing Windows 10 on an external drive can be found as several answers to the question: Is it possible to use Boot Camp with Windows 10 from an external HDD? 

Personally, I prefer my answer which only requires the external drive and the Windows 10 ISO file.