For those interested, here is a way to install Debian and boot it with UEFI if you have an UEFI motherboard. Using UEFI with Debian requires expert knowledge so if you do not feel up to it, keep your BIOS system.

1 Background

UEFI is a specification for motherboard's firmwares which is replacing the old BIOS. For now, UEFI motherboards still include a BIOS compatibility layer.

The UEFI boot process is quite different from the BIOS one. It involves one specific piece of the motherboard's firmware, the UEFI Boot Manager, which is able to load boot loaders from FAT file systems on specially-typed partitions. It can offer a boot menu (boot: Debian from HDD, Windows from HDD, USB stick, DVD?), which can be configured from a running operating system.

So, basically, to boot a system with UEFI, you need two things:

to install an UEFI boot loader on a FAT-formated EFI System Partition;

to tell the UEFI Boot Manager to create an entry for that boot loader.

2 Installation or preparation

If you are installing a new Debian system, read the first part. If you already have a BIOS-bootable Debian system installed, read the second one.

In both cases you will have to use Debian testing, because stable's GRUB has problems with UEFI. You can also use Debian stable while taking only GRUB from testing though.

2.1 Installation of a new Debian system

There is no installer image for UEFI, so you can either:

make your own, by installing an UEFI boot loader to an USB stick, which will load a regular installer kernel and initrd;

use a regular BIOS installer image, using the compatibility mode of your motherboard.

The installer does not provide a convenient way to install an UEFI boot loader, so you are going to install a regular BIOS boot loader at first, and switch to UEFI later.

Use the expert mode and format your hard drive with a GUID Partition Table (GPT). Create a small partition (1 MiB would be far enough), type it as a BIOS Boot Partition (this is the untitled flag above the “bootable” one in Partman), do not format it and do not mount it: this will be needed for BIOS booting. Create another small partition (same kind of size), type it as an EFI System Partition (this is the“bootable” flag), format it as FAT and mount it on /boot/efi: this will be needed for UEFI booting.

Finish the installation as usual, installing GRUB for BIOS, and boot your new system.

2.2 Preparation of an existing Debian system for UEFI

You need to use a GUID Partition Table on your hard drive. If you used an MBR, you lost; you may try to convert it to GPT using a tool such as gdisk, but that means operating without a net, loosing your current boot loader in the process.

If there is no space left on your hard drive, use Debian Live to make some. Create a small partition (1 MiB would be far enough), type it as an EFI System Partition, format it as FAT and mount it (permanently, using the fstab) on /boot/efi (that directory will not exist, so create it first).

3 Switch to GRUB UEFI

Install the package grub-efi-amd64. Prepare it by running the command (assuming your hard drive is /dev/sda):

# grub-install /dev/sda

That will do three things:

generate a GRUB image; install it to the EFI partition at efi/debian/grubx64.efi (relatively to this filesystem's root, which means /boot/efi/efi/debian/grubx64.efi in absolute); try to configure the UEFI Boot Manager (the motherboard's boot menu) to load it on start-up

That last step will fail. This is expected, because the UEFI Boot Manager can only be configured from an operating system that was started from UEFI. So if you stop here, you get an unbootable system.

Now, copy (a symlink would be relevant here, but that cannot be done on a FAT file system!) the GRUB image to that other path on the EFI System Partition: efi/boot/bootx64.efi (again, relatively to the ESP filesystem's root, which means /boot/efi/efi/boot/bootx64.efi in absolute). This is where the UEFI firmware looks for a boot loader when it has not been configured for a specific path, typically on removable media. Reboot to check that…