Trouble making a mirror of a slice.

[Nicholas Veeser]

I have two 250GB disks
I want to have a 20G Slice on each (with root and swap) and have a mirror.
Then I want the rest to be used in a ZFS pool.

I have ad4 and ad6

root is currently on ad6s1a
So I was going to try to create a mirror on ad4 and copy over to the
mirror and reboot from there.

So I tried...

# fdisk -p /dev/ad6 | fdisk -u /dev/ad4
# gmirror label -v -b round-robin gm0 /dev/ad4s1
Assertion failed: (md.md_provsize != 0), function mirror_label, file
/usr/src/sbin/geom/class/mirror/geom_mirror.c, line 245.

Any idea why this is not working?
Or why the assertion is coming up?
I suspect that there is something odd about my disk (data or hardware)
but I have no idea what it is.

n