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I have an external HDD that I have used for backup for many years. It has been formated MBR and gpt; and had partitions copied to it in both modes. Recently I discovered that if legacy mode is active, it will boot the legacy install. if legacy is inactive it will boot the efi install. I assume this effect is reproducible. however, since I stumbled on it by accident, I don't know what I did to cause it.

Thus, my question is: What steps do I do to make a HDD boot both modes? I'm making a diagnostic/recovery/install drive. will there be any limitations on grub booting other partitions? (most will be FAT32 for copies of diag disks)

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You need bootloaders for both modes.

  • The BIOS bootloader is the MBR in the head of the drive plus extra code in the rest of the first mibibyte of a drive with an MSDOS partition table and the content of a small partition with the flag bios_grub in a drive with a GPT.

  • The UEFI bootloader is the EFI partition and its content (a partition with a FAT partition table and a boot flag).

The following links provide detailed descriptions.

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