ywagmoth :
Currently windows 8 is on my C drive so if I install windows 10 to my D drive both will be functional? Will my windows 8 still work after I have installed windows 10?
you cannot do this, win 10 upgrade replaces your current install or its not an upgrade. You cannot use both at same time and the win 8 licence will become a win 10 one in a month anyway.
You can however fresh install win 10 using the
win 10 media creation tool to create an installer tomorrow and fresh install win 10 using your current win 8 licence when asked for a key during the install. Use one of the drives for storage, if you choose to use a new drive take the old one out during the install process or win 10 may decide to put files on it.
follow this:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html