Apple has updated their Time Machine system and it can now recognize a USB hard drive according to an article at The Register.

When users install the Time Machine and AirPort updates, their USB hard drives will now have an access to Time Machine’s capabilities like Apple’s Time Capsule hardware. The update can be downloaded through the Software Update functionality of OS X or users can download it at Apple’s support web site.

Previously, the Time Capsule hardware is the only computer hardware that can work with the Time Machine software.

Source: The Register