The option to install windows 7 in bootcamp blocked, help!

[Swagged]

I want to install windows 7 with bootcamp but I get this error message...

How do I resolve this? is it possible to install windows 7 on an external harddrive? if so, how?
Note: I have the yosemite Beta installed on another partition, I don't know if that's the culprit.

 

[saturnotaku]

is it possible to install windows 7 on an external harddrive?

Not possible, sorry.

 

[Swagged]

Not possible, sorry.

Oh, ok. How do I fix my current issue?