Possible Duplicate:
Failed to download repository information due to missing CDROM
W:Failed to fetch cdrom://Ubuntu 12.04 LTS Precise Pangolin - Release i386
(20120423)/dists/precise/main/binary-i386/Packages
Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
W:Failed to fetch cdrom://Ubuntu 12.04 LTS Precise Pangolin - Release i386
(20120423)/dists/precise/restricted/binary-i386/Packages
Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
E:Some index files failed to download. They have been ignored, or old ones used instead.
The above information is a crash info. It happens when I manually update Ubuntu. I have marked all checkboxes in settings that weren't marked by default.
What is the reason for this problem?