[gentoo-user] Wiping the old root without killing the new root

Hi,

Finally I moved to my new root and it seems to be $HOME
enough to wiupe the old root.

The old root is on a separate partition to which I will move
the contents of the new root after wiping the new root.

May be the following question is born from to much worry, but...

First I thought: Mount the old root to a certain mountpoint
somewhere, cd into it (as root) and do a rm -rf....

Then I saw symlinks directly pointing to /usr/lib... (for example)
right into my new root...

What is a recommended way to do what I am trying to do without
a) deleting anything outside the old root
b) doing it not TOOO SLOW
c) without leaving filesystem debris somewhere (for example after
   a dd if=/dev/zero of=/dev/sda2 count=1 bs=4096
d) anything else I forgot to think about

?

Thanks a lot for any idea! :)
Cheers
Meino