Re: Improperly nested partitions, help needed!

[Svante Signell]

On Tue, 2010-09-21 at 10:54 -0400, Lennart Sorensen wrote:
> On Tue, Sep 21, 2010 at 09:31:16AM +0200, Svante Signell wrote:
[...]

> Is there any chance you used to have an extended partition as sda3 and
> then made a linux partition which is now sda3 and your new extended
> partition is sda4?

Yes, if I remember correctly, that's what I did some time ago, when
installing a new disk for /home.

> If so, then there is a chance that the first sector of sda3 contains
> the old extended partition table, and grub probe might be detecting that.
> 
> One way to check would be to do:
> 
> dd if=/dev/sda3 of=/tmp/sda3.mbr bs=512 count=1
> 
> Then run 'file /tmp/sda3.mbr'.  If it says partition table, then I think
> you have found your problem.

Yes, you are right:
# file /tmp/sda3.mbr 
/tmp/sda3.mbr: x86 boot sector; GRand Unified Bootloader, stage1 version
0x3, 1st sector stage2 0xdd29b38; partition 1: ID=0x83, starthead 239,
startsector 63, 35153937 sectors, extended partition table (last)\011,
code offset 0x48

# fdisk -lu /dev/sda

Disk /dev/sda: 160.0 GB, 160041885696 bytes
240 heads, 63 sectors/track, 20673 cylinders, total 312581808 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xd568d568

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1              63     7575119     3787528+  1b  Hidden W95 FAT32
/dev/sda2   *     7575120   230640479   111532680    7  HPFS/NTFS
/dev/sda3       230640480   234541439     1950480   83  Linux
/dev/sda4       234541440   312575759    39017160    f  W95 Ext'd (LBA)
/dev/sda5       234541503   240861599     3160048+  83  Linux
/dev/sda6       240861663   261878399    10508368+  83  Linux
/dev/sda7       261878463   310413599    24267568+  83  Linux
/dev/sda8       310413663   312575759     1081048+ 82 Linux swap/Solaris

> If that is what is happening, then it is a question of whether grub-probe
> made a mistake when it scanned for partitions in something not marked
> as a valid place to look for partitions.  Of course the quick workaround
> in that case is to clear that sector (assuming the filesystem doesn't
> use the first sector of the partition, which many filesystems avoid
> since it is a handy place for boot loaders and such).

You are the Grub developers, you can tell if Grub does the right thing.

> If the filesystem does avoid that part of the partition, that would be a
> good reason an old extended partition table would have survived creating
> a filesystem.

So the solution is to blank out the first sector of /dev/sda3,

dd if=/dev/null of=/tmp/sda3 bs=512 count=1

to get rid of the warnings? Please confirm the command above is correct.

Thanks Grégoire and Lennart for your help!