Can I install Windows using a hard drive that has things in it? Or does it have to be 100% clean

[zhoos]

I have an external HD with music and videos on it. I want to use it to install windows 8.1 Do i need to wipe the external HD first?

 

[USAFRet]

Use it to install?
Just go get an 8GB USB stick and do it with that.

What you are trying to do is a maybe, but fraught with potential loss of all your stuff. A $10 USB stick will completely prevent that.