Missing Operating System after install of ubuntu on a macbook as only os

Question

I have installed Ubuntu on a 2009 MacBookPro (intel 64 bits) using the iso ubuntu-16.04.1-desktop-amd64.iso and unetbootin to create a live USB. So basically, I followed the tutorial to create a usb stick on mac found in ubuntu.com.

What I want to do is to have a computer running ubuntu and only ubuntu (so no other dual boot with mac). I therefore selected the option "install ubuntu on entire drive". The installation went smoothly so I was hopeful... But then when I restarted the computer : "Missing Operating System". I have no problem booting on the USB.

After some digging on the internet I figured that it might be due to an incorrect booting disk, so I executed the command : sudo fdisk -l which output the ram spaces and the two sda and sdb disks.

Disk /dev/sda: 465.8 GiB, 500107862016 bytes, 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
Disklabel type: gpt
Disk identifier: 2A269ECF-EA04-41B0-BAB9-FC531CC2D41A

Device       Start       End   Sectors   Size Type
/dev/sda1     2048   1050623   1048576   512M EFI System
/dev/sda2  1050624   2050047    999424   488M Linux filesystem
/dev/sda3  2050048 976771071 974721024 464.8G Linux filesystem




Disk /dev/sdb: 7.5 GiB, 8002732032 bytes, 15630336 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
Disklabel type: dos
Disk identifier: 0x00000000

Device     Boot Start      End  Sectors  Size Id Type
/dev/sdb1  *       32 15630335 15630304  7.5G  b W95 FAT32

The first disk is the hard drive of this machine and the second one is the LiveUSB.

What the problem is, I think, is that the 488M Linux Filesystem is not flagged as bootable but honestly I have no clue about what the problem is.

I have already tried :

• Repair grub with boot repair (here is the log : http://paste2.org/A3F398LX) but that did nothing.
• Reinstalling grub from liveUSB (https://ubuntuforums.org/showthread.php?t=1581099O . The problem here is that when I do sudo grub-install --root-directory=/mnt /dev/sdX I get : grub-install error will not proceed with blocklists and from here on I don't know what to do...

Any help is appreciated, thanks in advance.