cannot open consumer

[Dru]

Following Ralf's how-to. Successfully ran:

gmirror label -v -n -h -b round-robin gm0s1 /dev/ad2s1

Get this message on gmirror load:
GEOM_MIRROR: Device gm0s1 created (id=1024834944).
GEOM_MIRROR: Device gm0s1: provider ad2s1 detected.
GEOM_MIRROR: Device gm0s1: provider ad2s1 activated.
GEOM_MIRROR: Device gm0s1: provider mirror/gm0s1 launched.
GEOM_MIRROR: Device gm0 created (id=1640242348).
GEOM_MIRROR: Cannot open consumer ad0 (error=1).
GEOM_MIRROR: Cannot add disk ad0 to gm0 (error=1).
GEOM_MIRROR: Device gm0 destroyed.

I'm assuming that's not good. If I ignore that and carry on and bsdlabel 
/dev/mirror/gm0s1 so it looks exactly like /dev/ad0s1 (except a has offset 
of 16 and a's size is decreased by 16), I again run into problems when I 
go to insert the first drive into the mirror:

gmirror insert gm0s1 /dev/ad0s1
Provider ad0s1 too small.

Which is frustrating seeing the 2 drives are identical hardware, fdisk'd 
identically and labelled identically (except the offset 16 thing).

Suggestions appreciated. Here's uname:

FreeBSD some.host.org 5.4-RC3 FreeBSD 5.4-RC3 #0: Fri Apr 22 09:02:55 EDT 
2005 root at some.host.org:/usr/obj/usr/src/sys/NEW i386

Dru