I can't update 12.04 LTS. The Update Manager shows this:
CD/DVD 'Ubuntu 12.04 LTS _Precise Pangolin_ - Release i386 (20120423)' is required
Please insert the above CD/DVD into the drive '/media/cdrom/' to install software packages from it.
The image I downloaded and burned to CD is from http://releases.ubuntu.com/12.04/
the PC (Intel x86) desktop CD version. I burned many CDs but the result is the same.
Best Answer
Go to Ubuntu Software Center > Edit > Software Sources and make following changes: Uncheck the Cdrom option in Ubuntu Software.Also make sure that you have marked all the other options.
Now go to Other Software and uncheck the Cdrom options.
Now try running
sudo apt-get update