With the threads on "Stupid question" and "Throw an hard drive" in d-u at the moment, this seems timely:
I have a drive which contains two Debian installations, one of which (B) was installed for BIOS booting, and the other (E) was installed with EFI booting. I would like to convert B into a EFI system too. ¹ Each system has its own Grub installation, and os-prober has inserted entries for both systems into both systems. Consequently this means that I can boot in EFI-mode and run B, and I can boot in BIOS-mode and run E. As far as I can tell, the process for B to EFI should just involve: . boot B in EFI mode . mkdir /boot/efi/ . add it to /etc/fstab . mount the ESP onto it . remove grub-pc, grub-pc-bin . install (with Recommends) grub-efi-amd64, grub-efi-amd64-bin, grub-efi-amd64-signed, shim-signed, shim-signed-common, shim-helpers-amd64-signed, shim-unsigned . run update-grub . run grub-install (which should now modify /boot/efi, not the MBR). Is anything else required for B to become a "native EFI" installation? This conversion process will, I think, make the system boot into the EFI-ed B by default. If I want to make E boot by default again, should I boot E and run update-grub and grub-install?³ Or should I do this by running efibootmgr? -- ¹ More details: B was installed for dual-booting with EFI Windows, by switching into the BIOS (CMS) for booting B. Since then, Windows and almost all its entrails (various recovery, data and reserved partitions) ² have been overwritten with Debian installation E. The disk has all the necessary components: GPT partitioning, with a protective MBR and BIOS boot partition for B's use, and the original ESP for E's (and B's future) use, plus shared /home (encrypted), and a random encrypted swap. ² The ESP still has a "fossil" EFI/Microsoft/Boot/bootmgfw.efi file which merely causes grub-efi (through os-prober) to write a useless (but benign) entry in grub.cfg. ³ With totally BIOS systems, if I wanted a system "A" to be the default system when booted, I could boot A and run update-grub and grub-install. This would preen A's grub.cfg, and make the protective MBR boot to it. Cheers, David.