How can I use an external hard drive as a secondary internal hard drive on OS X So I have a 2011 iMac that has a 500 GB. Hard drive inside. I could not buy a new computer but I bought a 2 TB. external hard drive that I connected using firewire, since it is almost as fast as firewire. 

What I wanted to do is to have it act as an internal hard drive, automatically storing any new downloads to it. I thought I could do this because an iMac has two different SATA ports, but only one is occupied by the 500 GB. hard drive. Is there any way I can root the hard drive from the firewire port to make it act as if it was internal, an/or connected to the second SATA port?
The only way to do this is to move your users home folder over to the firewire drive.

see: http://chris.pirillo.com/how-to-move-the-home-folder-in-os-x-and-why/

Of course, this also means the firewire drive will now always have to remain connected, and you will lose some speed due to the fact you are interfacing with firewire and not sata.