OK. The way I did this was to completely remove postgres installation and then re-install it manually on the other drive.
Have a look at this post:
Total uninstall/reinstall of pt3You won't need to uninstall PT3, but it talks you through the postgres process. You will need to re-install it manually on the other drive.
I've been told that it's better to do it a different way - see the 2nd last post in the
Database / Postgres FAQ