How to install Windows 10 on external SSD that currently runs OS X? I made the mistake of purchasing a MBP w/ only 128 GB of storage. When I ran out of space on the internal drive, I purchased a 512 GB external SSD, transferred all the data over to the SSD, and now run El Capitan on the external drive and use it as my main startup drive (in fact, the internal drive is completely formatted). 

How do I partition the external SSD into two drives, one that runs OS X and the other with Windows 10? The reason I don't want to set up windows on the internal drive is because I want 256 GB dedicated to windows, leaving the other 256 GB on the external drive for OS X. 

Boot Camp Assistant seems to only detect the internal drive, so I don't know how to properly partition the drive and install windows 10 on it. I've tried partitioning with Disk Utility as well, but when running Boot Camp again, it only allows me to partition the internal drive. Maybe there is a way to install Windows on the internal drive, and then transfer it over to a 256 GB partition on the external drive?

I'm very new to/unfamiliar with this kind of stuff, so I'm sorry if this is a poor question.

Thanks for your help
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