Even if I’m trying to update with update manager or apt, synaptic is not updating. I faced this weired problem in ubuntu 10.04.
There is a very simple solution to get rid out of this problem.
Just execute this command in your terminal.
sudo gedit /etc/apt/sources.list
remove the line which contains CD/DVD ROM and the next line (if any, without starting with #)
update again with
sudo apt-get update
If you are no too unlucky you are already done with updating. enjoy