Ordered a MP today with the stock HD option. I want to buy a second and possibly third hard drive to use as a dedicated scratch disk for Photoshop. I shoot 8MP RAW files, convert them to uncompressed TIFF files in C1 (Capture One), and then work with them in Photoshop, typically working with 3-4 layers at a time with 1-2 photos open. Around 100-200GB should be okay, right? What's my least expensive decent option? What specs should I look for to tell if a HD is compatible w/ the mac pro? Links would definitely be appreciated.