You can have an entire Operating System on a USB drive and be able to start your ( or any other ) PC from that USB. The process is easy and you will only need two things :
1. UnetBootin which is a free application you can download here : http://unetbootin.sourceforge.net/
2. The ISO image file of the operating system you want to create the USB drive with, in our example this is Ubuntu and you can find this here : http://www.ubuntu.com/download/desktop
Take a look at our video :
The processFirst, download Ubuntu image and store it in a folder you can remember of. Then, download UnetBootin. Plug into your PC a USB drive, a USB stick that is, not a USB external drive. Then open up ( run ) UnetBootin and select "Disk Image". Then click the three dots to your right and select the image file of Ubuntu you just downloaded. Then, select from the list of drives in UnetBootin, the letter of the drive your USB has assigned.
Click "OK" and then the application will start writing the USB drive with the bootable content.
Once the write is complete, you will be able to start your PC with this USB.