Before we start I have researched this question and no answers help me. I have a laptop where the internal drive has failed. I have installed ubuntu on an external usb 1tb hard drive and it has been working well. Last night I closed down and today I get message - no boot device found - I can boot from live cd usb stick and see all data on external drive. External drive is ok but just will not boot - i have tried fsck and boot-repair - how can I just reinstall grub and make this drive boot again - thanks
OK no replies or assistance here so I tried to re-install from a Clonezilla image. Clonezilla told me system was set to boot in EFI mode but no EFI partitions, my hard drive is ext4. I checked the BIOS and for some reason factory defaults had been reset I enabled the option to boot in Legacy mode and ensured first boot option was usb hard drive - all works well now. I answer my own question in the hope it may help others.