Hello I am trying to install ububtu on an external drive. I am a programmer that wants to start using linux. I installed the usb installer from the ubuntu website and followed all the steps . when I get to the part where I have to choose the disk space it says an error when moving forward "No root file system is defined". Ive been doing some research and I think that I have to partition the external drive but do not know how to do so. The problem is that I only want 20gb used from that external drive and I want the rest as a storage device. I am also a musician and use pro tools so I would like to keep all my files there but I dont want ubuntu on my main hardrive since the external one is portable and I can bring all my stuff with me. If somebody can please help me out I would be greatly appreceiated. -especially if you can outline the steps for me since Im a complete nooob at this.