I had left off where I started the copying process. That finished overnight so I just tweaked the grub.conf file and shutdown the system.

I moved the new hard drive to hda and installed it’s twin as hdc. And started her up again.

The PC fell at the second hurdle (the first was: does it have a boot sector). It printed “GRUB” and then hanged . Armed with a Gentoo Install CD, I set about rescuing my system:

mknod /dev/md0 b 9 0
mknod /dev/md1 b 9 1
mdadm --assemble /dev/md0 /dev/hda1 
mdadm --assemble /dev/md1 /dev/hda2
vgscan
vgchange -a y vg
mount /dev/vg/root /mnt/gentoo/
mount /dev/vg/var /mnt/gentoo/var
mount /dev/vg/opt /mnt/gentoo/opt
mount /dev/vg/home /mnt/gentoo/home
mount /dev/md0 /mnt/gentoo/boot

Now with the system accessible, I can fix grub:

grub
device (hd0) /dev/hda
root (hd0,0)
setup (hd0)
quit

Okay, so now I get the GRUB menu list, but no further.

Seems I should have followed http://gentoo-wiki.com/HOWTO_Install_Gentoo_on_an_LVM2_root_partition a bit closer. I should have setup and initial RAM disk with the LVM2 utilities, devices and a start up script. Grabbing lvm2create_initrd from the LVM distribution got me a suitable RAM disc, but I needed to recompile my kernel with loopback, RAM disk and initrd support.