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I have a laptop with only BIOS (Ubuntu) and a laptop with only UEFI (windows) and a install Ubuntu USB stick and a USB external harddrive.

The USB install stick boots on the two laptops, but the external drive where i installed Ubuntu with the BIOS laptop doesnt boot on the UEFI laptop.

On the external disk I made an fat32 partition with boot, esp flag and a partition ext 4 with legacy_boot flag. I also made on the external harddisk a directory EFI with BOOT (inside bootx64.efi) and grub directory (inside grubx64.efi).

The external disk works fine on the BIOS laptop but on the UEFI laptop (secure boot disabled) when it boots i see only a black screen with grub>

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Who helps me out ?gparted of external disk enter image description here

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It seems almost OK, but the names of the directories under EFI are critical. I assume your disk partitioning is legacy, so no explicit 2M bios-grub partition is necessary. UEFI and DOS partitioning is not the standard way (use GPT and an explicit bios-grub partition as well as an EFI partition), but really it makes no difference in your case.

Sample EFI partition directory layout from wherever you mount it.

  • /EFI/Boot/bootx64.efi -- This is the bootloader that is used, so make it a copy of shimx64.efi for secure boot, otherwise a copy of grubx64.efi (or just set up for Secure Boot, and it works both ways.

  • /EFI/Boot/grubx64.efi -- When bootx64.efi is a copy of shimx64.efi, put the signed grubx64.efi in the same directory.

  • /EFI/ubuntu /grub.cfg -- The grub stub configuration file.

Example of the grub.cfg stub file with the root on the eighth partition:

search.fs_uuid Your-UUID-here root hd0,gpt8 
set prefix=($root)'/boot/grub'
configfile $prefix/grub.cfg
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