Strange message from yum.

[Darr]

Besides the other tips people have pointed out, the 'local'
plugin for yum causes a copy of every package and update
it's ever installed to be saved in /etc/yum/plugins/local.

So I would look there. You might find about 6GB of files
in that subdir, depending on how long after fc12 was 
installed that you added the local plugin and how many
packages and updates it's installed since.  Even if you want
to keep the local plugin, you could delete multiple old
versions of updates (e.g. just keep the latest plus one older
so you can downgrade if you need to). The excess kernel
debuginfo files alone (~200MB each) would be over a GB.