Problem:

While trying to update Ubuntu using the command sudo apt-get update in terminal results in the following error:

W : Failed to fetch cdrom://Ubuntu 11.10 _Oneiric Ocelot_ – Release amd64 (20111012)/dists/oneiric/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
E: Some index files failed to download. They have been ignored, or old ones used instead.

Reason:

As you can see in the error, it tries to get some information from the CD-ROM. For some reason, CD-ROM is listed in the list of software sources from where it tries to update.

Solution:

1. Open Dash, and type Software Sources.
2. Click on “Software Sources” icon, “Software Sources” window will appear.
3. Click on ‘Ubuntu Software’ tab.
4. Tick OFF against an item: “cdrom:[Ubuntu 11.10 _Oneiric Ocelot_ – Release amd64 (20111012)]/dists/oneiric/main/binary-i386/”.
5. Click on ‘Close‘ button.
6. Open up Terminal, and try again sudo apt-get update.

I hope it helped you. Questions and suggestion along with a word of thanks is always welcomed. Cheers

Source: Ubuntu Forums