If you have multiple hard disks in a system, or if you want a different partition layout than the default, you must manually edit your disk layout.
The installer partitions one disk at a time, and you can’t easily bounce between multiple disks. To successfully use multiple disks, decide on your partitioning scheme before you start the installation, and write down exactly which partitions you want on which disks as specifically as possible.
My system has two 50GB hard disks. I plan to divide the disks like this:
Disk 1 . 1GB /, 1.2GB swap, 5GB /tmp, 1GB /usr/X11R6, 2GB /usr/src, 2GB /usr/obj, and everything else /home
Disk 2 . 1GB /altroot, 1.2GB swap, 6GB /var, 10GB /usr/local, and everything else /var/postgresql