You have 2-3 choices on a backup drive.
The quickest and easiest to implement, and the fastest in use, would be to add a second internal SATA drive to your system. Advantages include low cost, high speed, and ease of use. Disadvantages include essentially no upgrade path when the drive fills with backup sets other than replacing the drive with a bigger one, and no security against a system-wide disaster like a fire in the building or a theft of the box.
An alternative would be to use an external drive. If eSATA, you keep the speed advantages, but it will be a little more expensive. If USB, you utterly lose any speed advantage, but if you schedule your backups for no- (or low-) usage periods, that may not be an issue. Since an external drive can be carried offsite, you lose the system-wide failure problems, but you may add cost if you rotate a set of them to keep a drive offsite at all times.
An option that loses most of those disadvantages is to use an external hard drive dock. You just slide a conventional hard drive into it. If it's eSATA, once again you have full speed. Costs are higher than an internal drive, but not by very much. Expansion is as easy as getting another drive, and you can keep one offsite for added safety. The only disadvantage that comes to mind is durability of the plug-in connectors, but the dock can always be replaced.