grub can't find new F12 install

[sean darcy]

I have an old F9 production remote server. I put in a new hard disk - 
sdb - and installed F12.

I want to be able to boot from sda to both my old trusted F9 and the new 
F12.

But now booting to F12 fails, can't find the files.

sdb is partitioned with ext3 for /boot - sdb1 and the rest is LVM for /. 
All done by anaconda on the install.

When I boot to F9, I can mount sdb1 and see the grub folder. But if I 
got to grub, find fails to see the grub folder on sdb1:

grub> find /grub/device.map
find /grub/device.map
  (hd0,0)

But, weirdly, if I cp device.map to the top folder on sdb1, grub does 
see it:

  grub> find /device.map
find /device.map
  (hd1,0)

Grub gets the geometry right:

geometry (hd1)
drive 0x81: C/H/S = 9726/255/63, The number of sectors = 156250000, /dev/sdb
    Partition num: 0,  Filesystem type is ext2fs, partition type 0x83
    Partition num: 1,  Filesystem type unknown, partition type 0x8e

I've fsck'd sdb1.

Any help appreciated. I'm clueless.

sean