Ray has an iMac and when he downloads programs to it, it wants to install it on the internal hard drive. Leo says he can put the apps just about anywhere he wants just by dragging them to a different location. He should just install it, and then move the file and applications folder onto another hard drive.
He can also just move his home folder to a separate drive. That'll save him a ton of space. He'll need to go into the System Preference pane, right click on his name, and select "Advanced Options." He'll have to do it as an administrator, and then change where the home folder lives in "Advanced Settings."