Whenever I try to update my computer by opening the Update Manager I get this problem and it just closes. Can someone help me?
W:GPG error: http:/packages.medibuntu.org quantal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2EBC26B60C5A2783,
W:Failed to fetch http:/ppa.launchpad.net/ferramroberto/gsharkdown/ubuntu/dists/quantal/main/source/Sources 404 Not Found
W:Failed to fetch http://ppa.launchpad.net/ferramroberto/gsharkdown/ubuntu/dists/quantal/main/binary-i386/Packages 404 Not Found
E:Some index files failed to download. They have been ignored, or old ones used instead.
