Recovering the bootloader after reinstalling Windows 7 (debian stable)

mzzxx11 · 10-12-2014, 04:15 AM

I have a dual boot machine (Windows 7 and Debian).
W7 and debian are on the same HD but on diffferent partitions.
The debian partition is an LVM encrypted one.

The W7 needs reinstalling, and as I understand the process will overwrite the debian bootloader (grub).

Question: Is there a way to save the current bootloader and recover it after I've reinstalled W7, so I wont have to reinstall debian from scratch?.
I plan to re-install W7 on the same partition it is now, without overwriting the debian partition.

Thanks

Doc CPU · 10-12-2014, 05:17 AM

Hi there,

Quote:

Originally Posted by mzzxx11

I have a dual boot machine (Windows 7 and Debian).
W7 and debian are on the same HD but on diffferent partitions.
The debian partition is an LVM encrypted one.

The W7 needs reinstalling, and as I understand the process will overwrite the debian bootloader (grub).

Question: Is there a way to save the current bootloader and recover it after I've reinstalled W7, so I wont have to reinstall debian from scratch?.

yes, there is. You could just allow the Windows installer to overwrite your current bootloader, then, later, boot a Live Linux and just re-install GRUB from that live system.

If you want to do it the hard way, that's possible as well. But still, low-level work on the HDD always poses the risk of an accident, and so I strongly recommend to have a full backup before you start, just in case something happens!

USE AT YOR OWN RISK!

I assume you're using GRUB as a bootloader. GRUB is typically installed as two parts. One part occupies the MBR and a few of the following sectors before the beginning of the first partition, the other part is in /boot/grub of the Linux partition (or maybe in an extra partition that is mounted to /boot). During re-installation of Windows, the GRUB part inside the Linux partition remains unchanged, of course. So it should be sufficient to back up the sectors preceding the first partition and restore them later.
You have a Linux live system ready that you can boot off a CD or USB pen drive? Good.

Here's how I would do it.

From inside Linux (the currently installed system will be fine) issue the command
Code:
```
sudo fdisk -l
```
Note the starting sector of the first partition (/dev/sda1). On my system, it's 2048, but it may be different on yours, especially when your partitions are not aligned to 1MB-boundaries.
Run the following command, replacing <number> with the number you noted before.
Code:
```
sudo dd if=/dev/sda of=grub.bin bs=512 count=<number>
```
Be aware that it's /dev/sda here, not sda1!!
Copy the file to an external media.

That's the preliminaries. Now you can re-install Windows, which will flatten the existing GRUB and replace it with its own bootloader. Once Windows is running again, ...

Boot your Live Linux from CD or USB, and have the previously saved grub.bin file ready
Issue the following command:
Code:
```
sudo dd if=grub.bin of=/dev/sda bs=512 count=<number>
```
Again, make sure you don't write sda1, it's just sda.
Reboot your system. It should come up with the boot configuration exactly as it was before.

Again: That's pure theory, I haven't tested the procedure. Don't try it without having a verified backup for disaster recovery, just in case.

Quote:

Originally Posted by mzzxx11

I plan to re-install W7

Why are you going to do that in the first place? Are you experiencing trouble with your Windows installation?
If I were in your shoes, I'd rather try a system restore from inside Windows, or restore Windows from a previous known-good backup.

[X] Doc CPU

EDDY1 · 10-12-2014, 05:46 AM

Just in case you need to mount with live-cd, I would say practice this
http://ubuntuforums.org/showthread.php?t=940904
& chrooting
http://www.linuxquestions.org/questi...el-4175437864/

mzzxx11 · 10-12-2014, 06:14 AM

Tnx for the reply.

DocCPU: what is the step-by-step procedure for easy option?

Say I've reinstalled W7 & it overwritten the boot sector. Now what?

Doc CPU · 10-12-2014, 06:24 AM

Hi there,

Quote:

Originally Posted by mzzxx11

DocCPU: what is the step-by-step procedure for easy option?

Say I've reinstalled W7 & it overwritten the boot sector. Now what?

Try a Google search. The procedure is described on many sites on the internet. For example:

http://ubuntuguide.net/how-to-restor...ws-xpvistawin7
http://community.linuxmint.com/tutorial/view/245
http://howtoubuntu.org/how-to-repair...ubuntu-live-cd

Though these tutorials specifically refer to Ubuntu or Linux Mint here and there, the procedure is the same for their ancestor Debian. Actually, for any distro using GRUB.

[X] Doc CPU

EDDY1 · 10-12-2014, 06:54 AM

If you get network running on live-cd right before chroot command you need to copy /etc/resolv.conf

Quote:

cp /etc/resolv.conf /mnt/debian/etc/resolv.conf

The live-cd has to be the same architecture as OS.

mzzxx11 · 10-13-2014, 05:59 AM

my current partitions are:

Code:

fdisk -l
Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 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: 0x00077703

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048   314574847   157286400    7  HPFS/NTFS/exFAT
/dev/sda2       314574848   324339711     4882432    b  W95 FAT32
/dev/sda3       324339712   325316607      488448   83  Linux
/dev/sda4       325316608   976771071   325727232   83  Linux

Disk /dev/mapper/sda4_crypt: 333.5 GB, 333542588416 bytes
255 heads, 63 sectors/track, 40550 cylinders, total 651450368 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: 0x00000000

Disk /dev/mapper/sda4_crypt doesn't contain a valid partition table

Disk /dev/mapper/linuxroot: 20.0 GB, 19998441472 bytes
255 heads, 63 sectors/track, 2431 cylinders, total 39059456 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: 0x00000000

Disk /dev/mapper/linuxroot doesn't contain a valid partition table

Disk /dev/mapper/linuxswap: 4999 MB, 4999610368 bytes
255 heads, 63 sectors/track, 607 cylinders, total 9764864 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: 0x00000000

Disk /dev/mapper/linuxswap doesn't contain a valid partition table

Disk /dev/mapper/linuxhome: 308.5 GB, 308541390848 bytes
255 heads, 63 sectors/track, 37511 cylinders, total 602619904 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: 0x00000000

Disk /dev/mapper/linuxhome doesn't contain a valid partition table

what do I need to do after re-installing W7 to recover these?
The linux partitions are encrypted (LUKS)

tnx

EDDY1 · 10-13-2014, 09:39 AM

See post #3