change gmirror's provider from whole disk to slice

[Rong-En Fan]

Hi,

Followed by the tutorial available from http://people.freebsd.org/~rse/
I have created a mirrored system disk by using the whole disk
(e.g., ad0). But now I want to use the second method mentioned in
the url above, i.e., using a slice as provider (say ad0s1). My question
is how to revert the process? After that, I can following the second
method in rse@'s tutorial.

Thanks,
Rong-En Fan