On May 11, 2014, at 9:14 AM, Timothy Murphy <gayle...@eircom.net> wrote:

> I rarely use Windows on my dual-boot Fedora-20/KDE laptop,
> but I'd like to update Windows XP to Windows 7 now.
> 
> I found when I did this on a CentOS machine
> I was unable to get back to Linux,
> and had to re-install CentOS.
> I don't understand why, as I had saved and re-installed the MBR.

You saved and reinstalled it how? It should just be grub-install <dev> or 
grub2-install <dev> (CentOS and Fedora respectively).

> 
> Am I wrong in thinking that this should be sufficient?
> If not, what else do I need to save?

Windows 7 will step on the first 440 bytes of the MBR with its own boot strap 
code, so Windows will boot but GRUB will not load and thus you don't be able to 
get to Linux. So you need to have a Fedora 20 install disk handy. The easiest 
is either netinstall or DVD, because they have an anaconda rescue boot option 
under the troubleshooting menu. This rescue boot is nice because it finds the 
Linux system, and mounts all of its parts correctly at /mnt/sysimage. So all 
you have to do is:

chroot /mnt/sysimage
grub2-install --no-floppy /dev/sda

--no-floppy will go faster and it also avoids baking in a floppy driver into 
core.img allowing it to be smaller.


If you use a Live CD/DVD, then you have to find and mount all the parts of your 
Linux installation correctly, including bind mounting proc and dev, and maybe 
sys and run also, for the benefit of grub2-install.


Chris Murphy
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