Part 1 - Creating the installation media
3rd party installation media creation tools like Unetbootin are not creating the USB installation media properly in many cases. A tool to create a correctly working installation media is GNOME Disks, it's available in the repositories of nearly every Linux distribution.
Open Disks - select Restore Disk Image from the menu on the top right.
Choose the ISO file and the USB drive to write it to, then start restoring.
In case you have no access to a Linux operating system and have to do it from within Windows, you can create the installation media with the diskpart
tool from a running Windows system.
Open Command prompt as administrator and execute :
diskpart
list disk
select disk *
clean
create partition primary
active
format fs=fat32 quick
assign letter=**
exit
Note : * = number of USB drive | ** = select a free drive letter
Now mount the ISO file and copy the content to the USB disk.
Part 2 - Reinstalling the GRUB boot loader
Boot from the Ubuntu installation media - select the option Try Ubuntu without installing.
Once you are on the Live desktop ... open a terminal and execute the following commands.
Important information concerning the USB boot options :
Choose the USB entry with UEFI in front, in case the systems are installed in EFI mode and
the USB entry without UEFI in front, in case the systems are installed in legacy BIOS mode.
In case your computer has UEFI BIOS execute these commands :
sudo mount /dev/sdXXX /mnt
sudo mount /dev/sdXX /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
grub-install /dev/sdX
update-grub
exit
Note : sdX
= disk | sdXX
= EFI partition | sdXXX
= system partition
In case your computer has legacy BIOS execute these commands :
sudo mount /dev/sdXX /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sdX
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
update-grub
exit
Note : sdX
= disk | sdXX
= system partition
You can identify disk and partition numbers by using GParted (included in the installation media).
Additional information : Do not forget to disable hibernation and Fast startup in Windows !
Then shutdown the PC completely, do NOT restart - otherwise it will always start Windows.