This is a common bug on ubuntu 12.04.
Since the affected repositories are the unchanging distribution repositories, one approach to circumventing this problem is to run the following commands immediately before running
sudo apt-get update. Be sure to adjust them for the name of the archive site you use as well as the name of the release you are using. Not everyone will need all four commands--see the comments at the end of the commands. Don't use them for the development release.
# If you have enabled Ubuntu Universe
# If you have enabled sources
# If you have enabled both Universe and sources
These commands tell the system that these files are up-to-date by updating the time on the archive files. Don't run these commands on other repositories, or after the bug is fixed.
Even after the problem is fixed, it may make sense to consider how to reduce bandwidth when running apt-get if you run it regularly and have either a slow or an expensive Internet connection.
It may make sense to run the update less often or, equivalently,
choose a repository that is updated less often the main ones.
Many include the "source" repositories in their list of software
sources. This is useful for doing apt-get source packagename, but
that may be sort of old-fashioned.
Unless you get source every day, you might consider removing the
source setting, and enable it again and do an update only when you
want to download some source. The source of various packages is
always changing so the index to be downloaded is always changing,
Another workaround is to update only the interested Index file:
Most of the time, we update package list to add a PPA. so this is a workaround for that
Create a file as
put this line in the file.
How ? : Type these in a terminal
sudo gedit /etc/apt/apt.conf.d/99mysettings.
Explanation: It causes
apt-get not to delete the index files which are already downloaded.
Disable the repositories which you do not like to update now. You can go here to see how you can disable repositories .
apt-get update command in a terminal. wait till it ends.
Finally again enable all repositories which you have disabled a few minutes ago. You have now updated PPA index with all previous indexes.
Another Option is to switch to a mirror instead of using us.archive.ubuntu.com
There are many mirrors which doesn't update the index regularly( probable once in a week).I solved the problem by switching to that particular mirror.
you can locate a mirror near you and see if that helps.
Hope this will help.