I use it for two things
I have my external drive divided into two logical partitions. The large one is for backups of my C:, D:, and G: drives. The (much) smaller partition is used for the scratch drive for video editing. I currently have a 160 GB drive, but it is shortly being replaced with a 250 GB unit. I run two internal hard drives, one partitioned into C: and G:, the other into D:, E:, and F:. C: is for programs, including the operating system, D: is for data, and G: is for graphics, mainly images from my digital camera. Periodically, G: is backed up to CDs for more permanent storage and to free up disc space. E: is the scratch drive for Photoshop, and F: is exclusively for the Windows swap file.