I'd reccomend using a program such as gparted or qtparted, if I'm not mistaken qtparted is avaliable as a dsl package. You can then use it to look at your hard drive partitions, format if necesarry, and see what number your second hard drive is (aka hda1, sdb1, etc.) If it's SATA, it's probably sdb, if it's IDE, it's probably hdb. But make sure first!
Then, under the install thing, when it asks you what partition to install to, put the one of your second hard drive.
Alternately, you could try: