Hi! I've got a Late 2012 iMac 27". It's got only a 7200rpm disk. It's probably out of the question to install an SSD/Fusion Drive due to price/difficulty. I want the fastest possible disk for OS/applications when i edit RAW images. I'm therefore planning to boot/run applications from an external disk, via Thunderbolt (or USB 3.0 if it's faster). What's the best setup for this? An SSD-disk? Multiple ordinary disks in some kind of RAID configuration? Price: max $600-700. Advice on particular brands/disks to get or avoid? Thanks a lot!