some 'been there, done that' guru will likely show up soon, but till then, here's what I see as the challenge:
Set up the new drive and restore all files to the new partition. Your old root partition should have boot and usr mount points you can restore the boot and usr partitions to on the restore. In your new root, edit /etc/fstab to remove the /usr and /boot mounts. You will also need to replace the old drive partition UUID for the root partition with the new one.
Then you'll need to re-install GRUB - see http://tcl.leipper.org/2009/05/restoring-grub/
for a couple of links on how to do this. This is basically just running the GRUB install program with parameters that tell it the target install partition. Editing the grub menu just won't do it as the boot sector code has to be able to find the /boot partition that has the menu in it and other boot files. The way that code gets updated is via the GRUB install utility. (see 'info grub-install' )
good luck, have fun!