Dual boot doesn't work anymore

[Goh Lip]

On 05/07/2010 04:27 PM, M. Fioretti wrote:
> Greetings,
>
> A few weeks ago I put an extra hard disk on a friend's computer (who
> knows nothing about software of any sort) and installed Ubuntu KK on
> it. It was the default system, in dual boot (which worked) with the
> Windows install still present on the original drive.
>
> my friend has used Ubuntu since then and is happy with it. One week
> later the install, I visited him again and ran a system update.
>
> Yesterday the friend decided to boot Windows for a Windows-only game
> or something that is still there and now "dual boot doesn't work
> anymore" ( we couldn't talk more about this over the phone and almost
> surely won't have time until sunday for reasons irrelevant here).
>
> Sunday I'll go there and try to fix things back to normal. The
> question for the list is simply:
>
> which specific Ubuntu documentation, blog posts, whatever I should
> read _before_ visiting my friend, so that next sunday I go there with
> a clear idea of what could have happened and spend as little time as
> possible to fix the problem?
>
> Please note that I am NOT looking for generic, dual boot install
> tutorials, I know where those are. Consider that everything was fine,
> and broke, even if we didn't realize it immediately, only after an
> upgrade. What could it be?
>
> Sorry if this message IS a bit vague, but of course, without the PC in
> front of me and no clue of what the error message is...
>
> TIA,
> 	Marco

Please explain "dual boot doesn't work anymore".
If he can still boot into ubuntu (not windows presumably), then all 
needs to be done is at a terminal
sudo update-grub
sudo grub-install /dev/sda

If, as is likely, he cannot boot into ubuntu, then prepare a grub rescue 
cd when you see him on sunday.

To make a grub rescue cd, at terminal (you have grub2?-karmic or later?)
grub-mkrescue anyname.iso                {for karmic and earlier}
grub-mkrescue --output=anyname.iso       {for lucid}
Burn that image iso file to a cd.


On sunday, at your friend's computer, boot up the cd. At prompt, type

search -f /vmlinuz
Note the output as hd0,x

Then boot into ubuntu by typing
linux (hd0,x)/vmlinuz root=/dev/sdax ro     {note the sdax too}
initrd (hd0,x)/initrd.img
boot

{note the sdax too}
please substitute (hd0,x) and sdax with output from
"search -f /vmlinuz"
If output is say, hd0,3, then you should type
linux (hd0,3)/vmlinuz root=/dev/sda3 ro
initrd (hd0,3)/initrd.img
boot

When booted, at terminal type
sudo update-grub
sudo grub-install /dev/sda

Marco, try it out at your computer first to boot your ubuntu before 
going to your friend's house.

Regards - Goh Lip

If you are up to it,(and it is the better way), you can also do the 
following instead...

search --no-floppy --label --set <labelname>
linux /vmlinuz root=LABEL=<labelname> ro
initrd /initrd.img
boot

*or*
search --no-floppy --fs-uuid --set xxxxxxxxxxx
linux /vmlinuz root=UUID=xxxxxxxx ro
initrd /initrd.img
boot


*not recommended way* (but may work)
set root=(hd0,x)
chainloader +1
boot

*or*
configfile (hd0,x)/boot/grub/grub.cfg