When a Linux Mint release goes out of support, together with the Ubuntu release on which it is based, the Ubuntu packages become unavailable. This can be annoying for old fossils like me who stubbornly contue to use a release that has gone out of support, as I’m doing with Mint 14 (Nadia), based on Ubuntu 12.10, (Quantal Quetzal). (“Yeah, yeah; I’ll upgrade soon; I promise.”) Luckily, the out-of-support packages remain available in a different location.
So, in /etc/apt/source.list, I could simply replace all instances of archive.ubuntu.com with old-releases.ubuntu.com, so that my /etc/apt/source.list now look like this:
deb http://packages.linuxmint.com/ nadia main upstream import deb http://old-releases.ubuntu.com/ubuntu/ quantal main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ quantal-updates main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ quantal-security main restricted universe multiverse