Synaptic download location

You could just mount the partition to "/var/cache/apt/archives". If you don't run "apt-get clean" the downloaded packages should stay there. I'd recommend doing a backup of the directory first, creating a new one and then mount the partition:
sudo mv /var/cache/apt/archives /var/cache/apt/archives_bk
sudo mkdir /var/cache/apt/archives
sudo mount /dev/sdXX /var/cache/apt/archives/

Source