Booting the Raspberry Pi up to use a USB HDD will give you extra storage capacity and also provide you with a device that is faster and more robust when it comes to accessing data repeatedly. This is especially useful when running large MapReduce applications with Hadoop. The USB HDD should contain external power, if possible, as the max recommended current on the USB ports is 100 mA.
Follow the listed steps to set up your USB HDD and configure your SD card:
Plug in the USB device to one of your Raspberry Pi's USB port's; remember to leave the keyboard attached.
Using the Linux instructions from Chapter 2, Setting Up your Raspberry Pi Software and Hardware for Parallel Computing, grab the disk name and unmount the USB device. You can use the instruction at the following eLinux page to format your USB drive: