moving to gpart mirroring, one drive at a time

[Randy Bush]

i have an existing old-style glabel gmirror.  a drive, ada0 failed.  we
inserted a new drive as ada0.  as the old drive is a teensie 80g drive
and the new is a 500g, we figured to create gm1, dump/restore to it, and
then swap out the old working 80g for another 500 and mirror away.

so i went to the handbook.  20.4.3 Creating a Mirror with an Existing
Drive does not seem appropriate as it assumes no source mirror and equal
sized drives.

so i figured to do the dance of 20.4.2 Creating a Mirror with Two New
Disks, but with one disk and then mirror back.  but ...

# gmirror label -v gm1 /dev/ada0
Metadata value stored on /dev/ada0.
Done.

# gpart create -s MBR mirror/gm1
mirror/gm1 created

# gpart show mirror/gm1
=>       63  976773104  mirror/gm1  MBR  (465G)
         63  976773104              - free -  (465G)

# gpart create -s BSD mirror/gm1s1
gpart: arg0 'mirror/gm1s1': Invalid argument

whoopsie.

clue?

randy