27 December 2015
I recently damaged my Windows 7 installation upgrading to Windows 10. The root cause was my dual-boot configuration with Gentoo Linux. Due to having the EFI partition on a drive separate from the drive containing the Windows (C:\), the installation failed. The error message was entitled “Something Happened” with the contents “Windows 10 installation has failed.” This took a lot of time to debug, and unfortunately, using Bootrec and other provided tools on the Windows 10 installation medium did not resolve the issue.
Here are the steps I followed to back up my Linux data.
Created a LVM (Logical Volume Management) snapshot volume proving a stable image for the back up. The L parameter specifies how much space to set aside for filesystem writes that happen during the back up.
# lvcreate -L512M -s -n bk_home /dev/mapper/st-home
Mounted the snapshot volume. As I was using XFS, I needed to specify the nouuid option or the mount would fail with a bad superblock.
# mount /dev/st/bk_home /mnt -onouuid,ro
Used tar to back up the directory and piped the output to GPG to encrypt the contents (as this will be going to a external HDD not covered under my LUKS encrypted volume). Because this back up was only stored temporarily, I opted for symmetric encryption to simplify the process.
# tar -cv /mnt | gpg -c -o /media/HDD/st-home.tar.gpg
The above was repeated for each of my logical volumes.
After the backup completed, I removed the snapshot volumes.
# umount /mnt
# lvremove /dev/st/bk_home
I then created a checksum to be used later.
$ sha1sum /media/HDD/*.xz.gpg > checksum.txt
Next, I formatted both of my harddisks and let Windows partition my SSD as appropriate. According to this Microsoft article, Windows by default will create a partition layout as follows.
1. EFI partition [> 100MB]
2. Microsoft reserved partition [16MB]
3. Placeholder for utility partitions
4. Windows partition [> 20GB]
5. Recovery tools partition [300MB]
Because I wanted both Windows and the Linux root filesystem to exist on the same drive, I added a boot partition and a large LVM partition in the placeholder, resulting in the following scheme:
1. [256MB] EFI
2. [16MB] Microsoft reserved
3. [256MB] /boot
4. [192GB] LVM
5. [8GB] Free space
6. [192GB] Windows partition
7. [300MB] Recovery tools
8. Free space
Recovering my Linux configuration was as simple as booting from the Gentoo live CD, installing Grub to the EFI partition, and restoring the partitions from the snapshot.