When using apt-get, how to control where the program installs
 

I have two hard drives. One of them is getting full, the other has plenty of space. How do I get apt-get install to install to the less-full hard drive? It defaults now to the close to full hard drive, which is the drive that has / on it.

You don't.

You could move & symlink /usr or something, but that's not recommended since if the other hard drive is ever unaccessible you run the risk of becoming unbootable.