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>