The easiest and most reliable way to run Raspbian on a USB device is to leave an SD card containing Raspbian in place, but use it only for starting Raspbian that is residing on a USB device. While setting up such a configuration is not rocket science, it can be confusing to a newcomer or someone unfamiliar with Linux internals. In an effort to simplify the task, I’ve created the attached script named ‘usb-boot’ to automate the process.
Source: Running Raspbian from USB Devices : Made Easy – Raspberry Pi Forums