5.0 rc2+ raid1 setup - booting off raid not working

[Carl Brewer <carl%bl.echidna.id.au@localhost>]

amd64 system (Intel C2D E7400).  Gigabyte GA-G31M-ES2L motherboard, 2 x 
320GB SATAII HDD's. NetBSD 5.0 from 20090307 iso (rc2 + a bit I guess)

I've setup raid1 on wd1 after installing on wd0 as one would normally 
do, following the raid1 root setup guide (and skipping the swap/kernel 
dump section, it's 5.0rc2+)

raidctl -s raid0
Components:
          component0: failed
           /dev/wd1a: optimal
No spares.
component0 status is: failed.  Skipping label.
Component label for /dev/wd1a:
   Row: 0, Column: 0, Num Rows: 0, Num Columns: 0
   Version: 0, Serial Number: 0, Mod Counter: 53
   Clean: No, Status: 0
   sectPerSU: 0, SUsPerPU: 0, SUsPerRU: 0
   Queue size: 0, blocksize: 0, numBlocks: 0
   RAID Level:
   Autoconfig: Yes
   Root partition: Yes
   Last configured as: raid0
Parity status: DIRTY
Reconstruction is 100% complete.
Parity Re-write is 100% complete.
Copyback is 100% complete.


Looks ok to me from here. I pax'ed over all the filesystem etc, and ran 
raidctl -A root raid0 so the array is persistent etc. fsck reports that 
it's ok :

 fsck -fy /dev/rraid0a
** /dev/rraid0a
** File system is already clean
** Last Mounted on /mnt
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
238562 files, 1345076 used, 150436143 free (2047 frags, 18804262 blocks, 
0.0% fragmentation)

The filesystem is all good and full :

newbox# df
Filesystem   1K-blocks       Used      Avail %Cap Mounted on
/dev/wd0a     303561438    2690240  285693128   0% /
kernfs                1          1          0 100% /kern
ptyfs                 1          1          0 100% /dev/pts
procfs                4          4          0 100% /proc
/dev/raid0a   303562438    2690152  285694166   0% /mnt

/mnt/etc/fstab :

# NetBSD /etc/fstab
# See /usr/share/examples/fstab/ for more examples.
#/dev/wd0a              /       ffs     rw               1 1
/dev/raid0a             /       ffs     rw               1 1
#/dev/wd0b              none    swap    sw               0 0
/dev/raid0b             none    swap    sw               0 0
kernfs          /kern   kernfs  rw
ptyfs           /dev/pts        ptyfs   rw
procfs          /proc   procfs  rw
/dev/cd0a               /cdrom  cd9660  ro,noauto

This was the installboot that I ran :

/usr/sbin/installboot -o timeout=7 -v /dev/rwd1a /usr/mdec/bootxx_ffsv1

my raid.conf file :

START array
1 2 0

START disks
absent
/dev/wd1a

START layout
128 1 1 1

START queue
fifo 100

When I switch the boot drive order in the BIOS the machine boots NetBSD, 
gives me no boot delay (from installboot, I would expect 7 seconds and a 
countdown) and then stops, saying this :

boot device: wd1
root on wd1a dumps on wd1b
no file system for wd1 (dev 0x10)
cannot mount root, error = 79
root device (default wd1a) :

Any suggestions?  Shouldn't it be trying to get raid0a as the root device?