Yes! Otherwise there is a risk that windows installs the boot manager on the HDD. That happens if the HDD is the first disk recognized by the bios. After installation, attach the HDD, goto disk management, create a partition and format it.
What program do you use? I have only a 60GB SSD and win7, M$ Office and all the other stuff on it.
Just make sure, you move your "my documents" folder to the HDD: right click on it in explorer, goto properties and the path tab.