My Book Studio 1tb or My Book Studio 2, 2tb?

I am photographer looking to buy another external hard drive for file back up and possibly to use as a fast scratch disk.

I have a Mac Pro 2.5 intel, 4gb RAM and I’m replacing my internal fujitsu drive @ 5400 rpm for a Scorpio Black 320gb @ 7200 rpm.  After that I thought about buying the My Book Studio 2tb with RAID to use one drive for photos and the second as a temporary back up and scratch disk for CS4.  Can I use a 1tb drive on the Studio 2 as a scratch disk and the other for photos?  Does anyone know if Adobe Photoshop will recognize the My Book Studio 2, 2tb as one drive or two drives?  I don’t know what partitioning is, but can I partition a part of  the My Book Studio, 1tb, firewire 800 drive to use as a scratch disk and partition the rest to use as photo file storage?  Just so you know I don’t rely on any brand of hard drive for secure back up but instead back everything up to DVD+RW’s and DVD+R’s.  Thank’s for all your help and feedback in advance.