Repair grub2 after upgrading windows 8.1 to 10 (odd situation)

Question

I know this is a common issue, but I think I'm facing a different situation in my notebook. It came with windows 8.1, I used a ubuntu 14.04 disk to install it. It worked for months this way: With bios in normal (efi) mode, it booted windows automatically. With bios in legacy mode, it booted grub2 and then ubuntu (there was an windows 8.1 option in grub2 menu, but it didn't boot, windows complained about efi).

This week I updated windows 8.1 to 10, apparently it messed a little my partitions table, so my linux ext4 partition changed from sda8 to sda7.

Now when I boot my notebook in legacy mode it shows me grub rescue. I change those 2 lines using set command, from sda8 to sda7 and then it boots ubuntu properly. But I'm unable to change it permanently.

• Tried to use boot-repair but it complained about something (sorry, I can run it again if necessary);
• Tried to use grub-install (both from the ubuntu system running in my HD and from a live cd), but it gives me the following error:

sudo grub-install --root-directory=/mnt /dev/sda installing for i386-pc platform. grub-install: warning: this GPT partition label contains no BIOS Boot Partition; embedding won't be possible. grub-install: warning: Embedding is not possible. GRUB can only be installed in this setub by using blocklists. However, blocklists are unreliable and their use is discouraged.. grub-install: error: will not proceed with blocklists.

Sorry if there are typos, I copied it by hand. Gparted giver the following layout for my hd:

/dev/sda1 ntfs windows re tools 499MB hidden,diag
/dev/sda2 fat32 system 300MB boot
/dev/sda3 unknown  128MB msftres
/dev/sda4 ntfs  446GB msftdata
/dev/sda5 ntfs  852MB hidden,diag
/dev/sda6 linux-swap  3.8GB 
/dev/sda7 ext4  463GB
/dev/sda8 ntfs samsung_rec2 15GB hidden,diag
/dev/sda9 fat32 samsung_rec 673MB hidden,diag

===================== There's the output of blkid and my fstab (using a live cd)

ubuntu@ubuntu:~$ blkid 
/dev/sr0: LABEL="Ubuntu 14.04 LTS i386" TYPE="iso9660" 
ubuntu@ubuntu:~$ sudo blkid
/dev/loop0: TYPE="squashfs" 
/dev/sda1: LABEL="Windows RE tools" UUID="CA8E20468E202CFF" TYPE="ntfs" 
/dev/sda2: LABEL="SYSTEM" UUID="0A23-2EAF" TYPE="vfat" 
/dev/sda4: UUID="CC0824CC0824B6FC" TYPE="ntfs" 
/dev/sda5: UUID="3282616E8261380D" TYPE="ntfs" 
/dev/sda6: UUID="35f64a04-1d06-43d9-8948-56266b1f00b3" TYPE="swap" 
/dev/sda7: UUID="ef284dc0-d070-460f-adc4-7ce3c8ba607a" TYPE="ext4" 
/dev/sda8: LABEL="SAMSUNG_REC2" UUID="02F4CB2EF4CB22B1" TYPE="ntfs" 
/dev/sda9: LABEL="SAMSUNG_REC" UUID="1028-2B57" TYPE="vfat" 
/dev/sr0: LABEL="Ubuntu 14.04 LTS i386" TYPE="iso9660" 
ubuntu@ubuntu:~$ cat /mnt/etc/fstab
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda8 during installation
/dev/sda7 /               ext4    errors=remount-ro 0       1
# swap was on /dev/sda7 during installation
/dev/sda6 none            swap    sw              0       0

Answer 1

Boot from Ubuntu install media - select Try Ubuntu without installing.

When the Ubuntu desktop appears - open a terminal and execute :

sudo blkid  

Mount the system partition - open another terminal and execute :

sudo mount /dev/sda7 /mnt  

Edit the fstab file - open another terminal and execute :

sudo gedit /mnt/etc/fstab

Replace the UUID entries with the ones from the output of blkid.

Note :

You should consider to install the recommended 64 bit version of Ubuntu in EFI mode, because the disk has a GPT partition table, and then you do not have to switch from EFI mode to Legacy mode in BIOS anymore. GRUB will be installed to the EFI partition where all boot loaders reside.

Update :

Repeat the procedure, edit the fstab file as follows and don't forget to save the file afterwards :

# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda7 during installation
UUID=ef284dc0-d070-460f-adc4-7ce3c8ba607a /               ext4    errors=remount-ro 0       1
# swap was on /dev/sda6 during installation
UUID=35f64a04-1d06-43d9-8948-56266b1f00b3 none            swap    sw              0       0  

In case it doesn't work, you have to live with this situation until you install Ubuntu 64 bit version.