[ubuntu] [permanent usb] impossible to install programs

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http://ubuntuforums.org – Hi all, I hope I'm in the correct section because I never used linux and this forum too. I just made a permanent ubuntu usb by using universal USB installer from windows. Following the program steps I installed ubuntu on a 32 gb usb with a casper rw to save settings and what I have done. Everything seems to work correctly. Now I would like to install some programs and keep them always with me. However when I try to do it I get the following message: ubuntu@ubuntu:~$ sudo apt-get install gnuplot-x11 Reading package lists... Done Building dependency tree Reading state information... Don (Hardware)