preupgrade f16 to f17: can't find mainimage path in .treeinfo: /dev/root does not exist

[John Pilkington]

Hi:  This box is running f16.  It has been upgraded in stages from f10. 
  Two disks, and when last tried could boot Win Vista, Vista recovery, 
f12 and f16.  It usually runs MythTV under f16, 24/7.

Preupgrade appeared to run ok, with the expectation of downloading the 
installer on reboot, as previously. Curl reported error 404, with later 
error messages as in the Subject.

http://www.mirrorservice.org/sites/dl.fedoraproject.org/pub/fedora/linux/...

I have saved a download of squashfs.img

Earlier this year dracut reported that ...lv_root contained a file 
system with errors, inodes 'part of a corrupted orphan linked list', but 
things still seemed to work.  Eventually I ran fsck on ..lv_root. 
Changes were made and the boot complaints stopped.

Last month, prompted by a post here that I can't now find, I ran fdisk 
-l and saved the result.  It sees /dev/sda and /dev/sdb, and one 
partition on each is marked as bootable, but none of the lvm volumes 
contains a valid partition table.  I've looked for grub and grub2 files, 
and although grub.conf has a recent date its contents are pre-f16.

I really don't want to wipe everything and start again.  Help please.

TIA

John P