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GRUB's GPT requirement
BIOS systems: when GRUB is to be installed on an HDD with GPT partitions, a bios_grub partition is required. We normally use the free space left before block 2048 when gdisk's default partition alignment is used.
EFI systems: when GRUB is to be installed on a UEFI system, an "EFI System Partition (ESP)" is required (a.k.a "EFI boot partition" by Debian and Ubuntu). It has to be big enough to contain all the boot loaders that will be installed in it, say 100 MB for a single OS system and 200 MB for multio boot.
Which partition management utility to use?
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