Error "Media change: please insert disc labeled..." when installing an application [duplicate]

Probably you still have the following option ticked in your Software Sources:

If ticked, Ubuntu will install packages from the installation media rather than downloading from the internet.

This happened to me on 12.04 (precise).

What got me into this mess in the first place was trying to upgrade from 11.10 to 12.04 by cdrom.

We must be careful here, whereas it's the "Other software" tab that we enable the cdrom on, it's the "Ubuntu software" tab that we need to disable.

Naturally you can also try this:

1. edit the file /etc/apt/sources.lst -> comment out the lines referring to cdrom by removing the # in front of it
2. Then execute the command in a terminal sudo apt-get update

but know what you're doing!