If your device is IOS 4.2.1 , ubuntu maybe can't mount success .

1. vi /etc/apt/sources.list

 deb http://ppa.launchpad.net/pmcenery/ppa/ubuntu maverick main
 deb-src http://ppa.launchpad.net/pmcenery/ppa/ubuntu maverick main

2. apt-key adv --keyserver keyserver.ubuntu.com --recv-keys D48B8E25
3. apt-get update && sudo apt-get upgrade
4. try it . !