Thanks everyone, for your responses. I have bought a 2tb hdd and have installed it, I have already copied my backup files to it. I have decided to wipe the whole 600gb hdd and install windows 7 back on the drive without partitioning it. I have windows 7 installed on a 146gb partition at the moment and have problems with space.
I will definately unplug the new hdd when I re-install windows on the other drive, will there be any problems with this hdd when I plug it back in as it will have all the backed up stuff I need on it?