I always use multiple hard drives for different groups of images. Some are eSATA, some are Firewire800. I even use network drives. I find by creating smaller libraries on each drive for each group of images, Lightroom is much faster. The one caveat is that a library for a network drive has to reside on your local hard drive. For that, I've created a separate folder for misc. Lightroom libraries. For me, at least, it's crazy to have a million images in one Lightroom library. For me, this way, each job, each project, has it's own Lightroom set of files. For instance, on one drive I keep all my portfolio images. On another, I have all my images from Ireland, on another all other travel related images. I use an SSD drive for my swap drive.