Check out Samsung's T1 or the new T3 SSD external drives. The T3 comes in a 2 TB capacity plus smaller sizes, and the T1 is available in a 1 TB size as well as the smaller capacities. This is a very fast, physically small bus-driven USB 3.0 connection external drive that works well as a supplemental drive when one has a computer with limited SSD capacity. I bought a retina MacBook Pro (512 GB SSD) in December and shortly after setting it up also purchased a 1 TB Samsung T1 to use as a supplemental drive. I also have various other external drives that are used for backup purposes. The T1 or T3 are not inexpensive, but I think that the SSD speed and the incredibly small size, great for travel, makes it worthwhile.
If you're after more capacity than this, since you're using a desktop machine and portability might not be all that important to you, the Seagate Backup Plus drive comes in various capacities, I think up to 4 TB, perhaps more, and it again plugs into a USB 3.0 port.