Mandriva 2008

I tried this guide which is very detailed. I know I had tried this before some months ago an it didn't seem to work then for reasons i can't remember or couldn't understand.
This time, I ran into the problem that I couldn't loop mount the initrd.img file. The reason for this is that 2-3 years ago, initrd files became cpio archives which don't support loop mounting. Instead a different technique is needed which is described here ( as is the older version).

Note that this line

# cpio -idv < ../initrd-2.6.9-42.ELsmp.img.gz

in the cpio method should in fact be

# cpio -idv < ../initrd-2.6.9-42.ELsmp.img

Anyway, having opened the unzipped initrd file and looked through its /lib directory it seemed to be missing two drivers that the first guide had insisted were necessary for usb booting. These were ohci-hcd.ko and usb_storage.ko.
So, I got both of them from the mandrive /lib/modules/kernel version/kernel/drivers/usb/ directory and copied them to the unzipped de-cpio'ed initrd module.

Then I zipped it up again as directed in the second guide and copied it as appropriate to get it to boot from the mbr on the Ubuntu root partition.
Then I tried to boot Mandriva. However, it kernel panicked exactly as it had done before suggesting that what i fixed wasn't the problem.
It really seems like it goes through the boot process in less than 5 seconds which seems so much faster than for other OSes. Not sure if this means not enough time is available for the usb drive to start up.
Anyway, I'll keep looking.