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After installing fresh Windows 10 on my Laptop, my external hard drive doesn't boot previously installed Ubuntu. Even though the grub menu appears at the beginning, after selecting ubuntu, the screen becomes black with a blinking console cursor. I reinstalled grub with live CD with no luck. After attempting Boot-Repair from live cd, here is the report in provides: http://paste.ubuntu.com/13297431/ It still not working. How can I get back my Ubuntu exactly same as before?

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Remove the FlexNet crap from the boot sector we found during our discussion in chat.
GRUB refused to install to the mbr complaining about a sector being in use by FlexNet.

Afterwards reinstall the GRUB boot loader to your Ubuntu installation in legacy mode.
Boot from your ubuntu installation media and choose Try Ubuntu without installing.

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

sudo dd if=/dev/zero of=/dev/sda bs=512 count=62 seek=1
sudo mount /dev/sdc2 /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sda

Note : sda = disk | sdc2 = Ubuntu system partition

In case this solution does not work, open GParted and shrink the Windows partition on sda.
Start Install Ubuntu - choose Something else and install Ubuntu to the unallocated space.

If you want to clone your old system to the new partition use clonezilla for this.
Download Clonezilla Live CD and then create a bootable media to boot from it.
Backup the Ubuntu partition from the external disk to another disk or partition.
Restore it back to the partition where you performed the new install of Ubuntu.

Download and more information -> Clonezilla

Now boot from Ubuntu install media again ...

Identify the partition UUID's - open a terminal and execute :

sudo blkid  

Mount the system partition and edit the fstab file - open another terminal and execute :

sudo mount /dev/sda* /mnt
sudo gedit /mnt/etc/fstab

Replace the UUID entries with the ones from the output given from the blkid command.

In case Ubuntu will not boot - reinstall GRUB ... boot from Ubuntu install media again ...

Open a terminal and execute :

sudo mount /dev/sda* /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sda  

Replace * with the partition number you have Ubuntu installed.

If all this is too complex or complicated - you as well can keep the working Ubuntu configuration on sda and copy your personal data from your old Ubuntu on sdc to your new installation on sda.

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    Since we don't see the chat, it may be a good idea to edit the question and explain the original problem in more detail if you think this problem is general enough to help others in similar situations. – user68186 Nov 27 '15 at 13:34
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    @user68186 : Good idea ... I updated the answer with the main point (mbr sector being in use) accordingly. Thank you for your contribution ! :) – cl-netbox Nov 27 '15 at 14:08

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