GRUB overwriting partition tables

[René Herman]

Good day.

Yesterday I installed the recent Zenwalk 6.2 beta:

http://www.zenwalk.org/modules/news/article.php?storyid=99

which is the first Zenwalk release to feature GRUB(2) instead of LILO. 
Unfortunately the installation, on a dedicated test partition, destroyed 
my system by breaking the partitioning.

More specifically, what happened is that not just the MBR (meaning 
sector 0 here) where I chose to install GRUB2 but also sector 1 got 
overwritten somehow.

Given an admittedly somewhat peculiar partitioning scheme, this had the 
very bad effect of breaking the chain of logical partitions, right at 
the start, and thereby rendering all logical partitions with my working 
OS and data inaccessible. I managed to restore sector 1, but really only 
because I also knew that my first logical was exactly 8G in size.

The trigger is most likely me not cylinder-aligning partitions. Nothing 
else in Linux insists on doing such and in fact, this is how "sfdisk" 
creates the chain directly. This is my partition layout:

   Device Boot    Start       End   #sectors  Id  System
/dev/sda1             1 692272486  692272486  85  Linux extended
/dev/sda2     692272487 976773167  284500681   7  HPFS/NTFS
/dev/sda3             0         -          0   0  Empty
/dev/sda4             0         -          0   0  Empty
/dev/sda5             2  16777217   16777216  82  Linux swap
/dev/sda6      16777219  33554434   16777216  83  Linux
/dev/sda7      33554436  50331651   16777216  83  Linux
/dev/sda8      50331653  67108868   16777216  83  Linux
/dev/sda9      67108870 379690677  312581808  83  Linux
/dev/sda10    379690679 692272486  312581808  83  Linux

Since I often have multiple operating systems installed -- at the moment 
only Linux and the Windows 7 RC but sometimes also MINIX or BSD variants 
-- all of which need at least 1 primary partition, I use the 0x85 "Linux 
Extended" type as "the one and only true" Linux partition, with the 
actual data partitions as "subpartitions" (or "slices" in BSD 
terminology) on logicals inside.

Linux is also my main system, so it's the first primary, meaning the 
chain of logicals also starts at the beginning of the disk. In this case:

MBR (sec0) --> EXT (sec1) --> 16777218 --> and so on.

After realizing what the problem was exactly, and knowing that /dev/sda5 
was exactly 16777216 sectors in size, I could restore sector 1 but until 
that point I wasn't particularly happy about seeing my partitions gone 
(as an aside, not cylinder aligning but making my partitions these exact 
easily remembered sizes has saved my butt several times while playing 
around with beta or old operating systems).

The Zenwalk 6.2 beta install gives the possibility to install GRUB into 
the MBR, into the bootblock of the install partition, /dev/sda7 in my 
case, and onto a device of your own choosing. I chose the MBR and after 
finishing up the installation, I rebooted only to be told by GRUB that 
it couldn't find sda7. After seeing what the problem was, I actually 
looked at sector 1 in a disk editor, and it indeed got overwritten.

I'm now in my main system on /dev/sda6, and am booting the beta system 
on /dev/sda7 with LILO, which works fine. Below is a copy of the 
"grub.cfg" as generated by the installer. GRUB version seems to be 
"grub2-svn2383".

Now that I know how to fix things again, I can if really needed repeat 
the process and gather any requested information while the system is 
hosing itself. If any is requested, please guide me what, and when in 
the process, it is needed.

Thanks in advance to anyone interested in getting to the bottom of this; 
I'd like to keep running zenwalk...

Rene.

== GRUB.CFG ==

#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by /usr/sbin/grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
set default=0
set timeout=5
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/10_linux ###
menuentry "Zenwalk GNU/Linux" {
        set root=
        linux16 /boot/vmlinuz root=/dev/sda7 ro   splash=silent 
resume=/dev/sda5 vga=794
        initrd16        /boot/initrd.splash
}
menuentry "Zenwalk GNU/Linux (recovery mode)" {
        set root=
        linux16 /boot/vmlinuz root=/dev/sda7 ro 1
        initrd16        /boot/initrd.splash
}
### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/30_os-prober ###
menuentry "zenwalk (on /dev/sda6)" {
        set root=
        linux /boot/zenwalk root=/dev/sda6 ro resume=/dev/sda5 
splash=silent
        initrd /boot/initrd.splash
}
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/40_custom ###
# This file is an example on how to add custom entries
### END /etc/grub.d/40_custom ###