[ubuntu] Failed To Download Repository Information "apt-cdrom"

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http://ubuntuforums.org – I am using Ubuntu 12.04 with Gnome Classic. To update, I go to Applications>System Tools>Administration>Update Manager and clickon [Check]. The check proceedure then commences. After a while, I get the following error message "Failed to download repository information" with the following details: Quote: 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 (Hardware)