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How is my MacBook Air which uses UEFI boot system is able to detect and boot Ubuntu 19.04 and how is my desktop PC with legacy BIOS boot system is also able to detect and boot the same exact installation of Ubuntu 19.04.

I manually installed Ubuntu 19.04 on a portable external hard drive and it can be detected by both MacBook Air and Desktop PC like mentioned above.

Here is the Ubuntu partition layout, using GPT partition scheme:

10MB of “Reserved BIOS Boot” 500MB of “/boot” 16GB of “swap memory” 3TB of “/root”

Can someone explain how is the grub system on Ubuntu 19.04 is able to boot from both UEFI and BIOS systems?

I installed Fedora 30 exactly the same way as Ubuntu in BIOS mode and it’s not showing up in my MacBook Air when I press the “alt option” key.

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  • It doesn't. You simply install two instance of grub, one for BIOS and one for UEFI Commented Sep 6, 2019 at 22:34
  • If you have a bios_grub partition which only needs to be 1 or 2MB, then you have a BIOS bootable install. Most UEFI systems will boot BIOS installs. Grub requires a bios_grub partition to correctly install to gpt partitioned drives.
    – oldfred
    Commented Sep 7, 2019 at 3:46
  • @oldfred Thank you for the useful info. Makes sense why UEFI systems is able to boot BIOS installs. Why isn't Fedora 30 (BIOS installed) able to be detected by my MacBook Air? Is their any tutorials on how to manually install grub so that it can be booted by both UEFI and BIOS systems? My guess is that Fedora 30 grub system is broken in BIOS installs.
    – S To
    Commented Sep 7, 2019 at 8:06
  • @oldfred Grub doesn’t need a bios_grub partition, at least not in the last few years. Commented Sep 7, 2019 at 8:43
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    Grub has to have bios_grub on gpt drives, not required on MBR drives for BIOS boot. But gpt has some advantages. For UEFI boot you need an ESP - efi system partition. When I got my first UEFI system I put both a bios_grub & ESP on every drive. If you force grub to install without bios_grub it uses blocklists which is highly NOT recommended.
    – oldfred
    Commented Sep 7, 2019 at 12:16

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