Color keeps cancelling render- reset "scratch drive"

Okay so I'm new to Color. Know Final Cut inside and out but not so much Color. How do you set your "scratch drives" in Color. I know it isn't called your scratch drive in Color but you get what I mean. My start up disk is full after all of my "Color documents" were put on it. I would like to put the stuff on my external MyBook which has right around 250 gigs left, but also have multiple hard drives and networks. Please help. Thanks.