How to install Windows 10 on external SSD that currently runs OS X?
As Apple states here:
https://support.apple.com/en-us/HT201456


  Can I install Windows on an external drive?
  
  No, Boot Camp requires that Windows be installed on your internal hard disk.



There's a workaround, which doesn't use Bootcamp. Personally, I haven't tried it.
http://bleeptobleep.blogspot.gr/2013/02/mac-install-windows-7-or-8-on-external.html