Talk:Linux Partition Table

EFI-mode booting from GPT

# efibootmgr
BootCurrent: 0000
Timeout: 0 seconds
BootOrder: 0000,0001,0002,0003,0004,0005,0006,0007
Boot0000* ubuntu
Boot0001  DTO UEFI USB Floppy/CD
Boot0002* DTO UEFI USB Hard Drive
Boot0003  DTO UEFI ATAPI CD-ROM Drive
Boot0004  CD/DVD Drive 
Boot0005* DTO Legacy USB Floppy/CD
Boot0006* Hard Drive
Boot0007* IBA GE Slot 00C8 v1550

BIOS-mode booting from GPT -- On a BIOS-based computer or an EFI-based computer that uses its Compatibility Support Module (CSM) to emulate a BIOS, GRUB is installed split across the Master Boot Record (MBR; the first sector of the disk), the BIOS Boot Partition, and files in the Linux /boot directory. In some cases, the BIOS Boot Partition code can be placed elsewhere, but Ubuntu pretty much insists on the BIOS Boot Partition being present. Since you don't have this partition on your disk (it shows up as having the "bios_grub flag" set in GParted), it's pretty clear that your disk is not set up for BIOS-mode booting.

EFI-mode booting from GPT -- Under EFI, booting is handled by boot loaders stored on the EFI System Partition (ESP), which shows up in GParted as having the "boot flag" set. The ESP is normally mounted at /boot/efi in Ubuntu, and Ubuntu's GRUB goes in the ESP's EFI/ubuntu directory -- that is, /boot/efi/EFI/ubuntu. The computer knows which boot loader to launch because of NVRAM entries, which you can examine and manipulate with the efibootmgr command in Ubuntu. Type sudo efibootmgr to see the existing entries, or add -v to see extended information. You can use -c to add new entries, -B to delete entries, -o to change the boot priority, and so on. (Some of these options require additional options to do any good.) Type man efibootmgr for details. Note that EFI boot loaders are ordinary files; they have no components that are stored "raw" in the MBR or partition boot sector, unlike BIOS boot loaders. EFI-mode GRUB still relies on files in the Linux /boot directory tree, though, just like its BIOS-mode counterpart.