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Comment by pabs3 | Hacker News Reader

Hmm, I wonder what to do if you want to be able to boot the same system via both BIOS and UEFI.

you can have both MBR and GPT on the same disk. GRUB explicitly supports it too, if it finds a EE00 partition type, it will parse the disk again as GPT and boot normally to boot a second stage of the ESP. A UEFI system will ignore the MBR and use the GPT table to boot directly into the ESP. The Gentoo wiki has a description how to set this up.

I’m curious why that would be useful?

Live images. Also if you still use BIOS-era hardware you found in the trash because you can't afford to buy hardware, and if someone gave you a UEFI device you can't afford the additional storage devices needed to migrate data and or because you don't want to have to deal with migrating a system or reinstalling it.

>what to do if you want to be able to boot the same system via both BIOS and UEFI.

UEFI is simply supposed to find an EFI folder containing boot files, on a FAT32 partition.

A proper UEFI firmware will find the EFI folder regardless of whether the layout is for MBR or if it is GPT.

So you just use conventional MBR layout, format with FAT32 and throw on a (carefully crafted) EFI folder.

This is how Windows live (startup) media is normally done, so you can boot to either type and install Windows to the PC. On recent Windows versions the install.wim file has crept up in size beyond what FAT32 will support, so NTFS is needed for them.

Linux live distros do real well booting from FAT32 using Syslinux (ntfs is also an advanced option), just like the monolithic ISO's do using Isolinux. But to install the distro on the HDD I want an additional EXTx partition to hold the Linux, and still boot to it from the FAT32 boot volume.

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