에러 메세지

Failed to open \EFI\BOOT\grubx64.efi - Not Found  
Failed to load image \EFI\BOOT\grubx64.efi: Not Found    
start_image() returned Not Found 

해결법

USB로 일단 부팅 후 아래의 명령 입력

# find the EFI partition
sudo fdisk -l 

# mount EFI
sudo mount /dev/<partition_name> /mnt

cp -p /cdrom/EFI/grubx64.efi /mnt/EFI/ubuntu

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출처:
https://askubuntu.com/questions/906776/error-failed-to-open-efi-boot-grubx64-efi-dual-booting