Ubuntu – How to point grub back to a working Ubuntu (btrfs !)


I have installed a second Ubuntu on laptop that had already a MSFT Windows and a working version of Ubuntu 18.04.
Upon rebooting, I realised grub did not show two but one Ubuntu. This new version would not let me log in.
The boot is a EFI one and, after informing myself, it seems that the issue is that under the EFI partition only one /ubuntu directory is created. So the grub.cfg is overwritten.
I would like as a first step to try to recuperate my standard Ubuntu OS.
I have tried several things to that intent:
1- I installed rEFInd. It is indicated that rEFInd would find all OS, regardless that they are EFI bootable or not. rEFInd is now my boot manager but the entry that it did not restore the partition I am looking for magically.

2- I run the default and advanced options of boot repair to no avail.

3- I have tried to chroot
How can I reinstall GRUB to the EFI partition?
After following all the instructions from boot-repair live disk, I get a final error I did not managed to overcome after trying grub-isntall: /boot/grub no such device (did you mount /dev ? –which I did, according to instructions).

I am now trying to modify the grub.cfg directly to point to the old partition. There is a fs UUID instructions with root hd0,gpt3 there,
that I would like to tweak but I am unclear how to do that.

Any help would be appreciated.


Best Answer

A lot of thanks to Pedro de Paula for keeping me in the right direction on several occasions.

I managed to recover my lost OS by chrooting. The reason why it failed on a previous attempt was that the file system is btrfs and it differs subtlely from extxxx.

The whole think is very well explained here:

http://logan.tw/posts/2015/05/17/grub-install-and-btrfs-root-file-system/ I strictly followed these instructions and it worked.

Hope it helps.