The CentOS 8 installer will not allow you to create GUID Partition Tables unless it is UEFI booted. By default, it is indicated as UEFI bootable via an MBR with an ESP, which means it must be written to an optical disc for UEFI boot. Although there is also a GPT, it does not properly indicate the ESP partition. And besides, most GPT tools expect the protective copy of the GPT to be at the end of the disk, not some other random location.
Fortunately, conversion to a bootable USB stick is easy.
-
Copy the ISO image to the USB stick via
dd. Make sure you save another copy of the first 512 byte sector for future reference.USBSTICK=/dev/sdb sudo dd bs=1M if=CentOS-8-boot.iso of=$USBSTICK dd bs=512 count=1 if=CentOS-8-boot.iso of=mbr.img -
Open the disk image with
gdiskand instruct it to convert the MBR to the GPT, ignoring the existing GPT.gdiskis an interactive command-line utility, follow the prompts, then typewto write the changes to disk and answer “yes” that you are sure. -
Copy the 446 bytes of boot code from the old MBR back to the new protective MBR.
sudo dd bs=512 count=1 if=$USBSTICK of=new.img dd conv=notrunc bs=1 count=446 if=mbr.img of=new.img dd bs=512 count=1 if=new.img of=$USBSTICK # Cleanup: rm new.img mbr.img
Now you’re destination USB stick should be bootable both with legacy BIOS boot and UEFI boot. This will make installing partitioning either as MBR or GPT a breeze: simply change how you boot the CentOS installer.
Unfortuantely, CentOS 7 and CentOS 8 have a defect after you finish installing from UEFI boot: the GRUB x86-64 EFI boot modules are missing from the installation. Install them manually as follows:
sudo yum -y install grub2-efi-x64-modules efibootmgr
20201210/Google error: /usr/lib/grub/x86_64-efi/modinfo.sh doesn’t exist.
20201210/Google EFI variables are not supported on this system.
20201210/https://unix.stackexchange.com/questions/574170/grub2-install-error-modinfo-sh-missing-common-solutions-attempted
Note, also, that the CentOS 8 installer has a defect that it doesn’t
allow you to create a BIOS boot partition to support legacy BIOS boot
on GPT. You can create this manually in the parted command line.
Keep in mind, however, that due to GRUB limitations, you must create
this partition reasonably close to the beginning of the disk or else
it will not work. For this reason, it is easiest to use parted to
create the partition before you install CentOS, and simply instruct
the CentOS installer to leave that partition be.
parted /dev/sda mkpart bios-boot fat32 1MB 2MB
GRUB only needs ESP to be 1 MB, since it is just a stub that loads the rest. BIOS boot partition only needs 32 KiB, but make it 1 MB for good measure.
Note that grub2-install will not automatically identify and
initialize that partition with boot code, unlike what one of my online
sources said. Only grub2-install --target=i386-pc will initialize
the BIOS boot code, and it will (essentially) fail if the BIOS boot
partition is not present on a GPT disk. So if you want to support
both UEFI boot and BIOS boot, you must run two grub2-install
commands as follows.
grub2-install --target=i386-pc $TARGETDISK
grub2-install --target=x86_64-efi $TARGETDISK
grub2-mkconfig -o /boot/grub2/grub.cfg
Also, if you want to install to a USB removable disk or the like for a
portable boot drive, you must use --removable --force for
both the legacy BIOS and the UEFI bootloaders.
One peculiarity that I have to look further into is that the CentOS 8
installer’s UEFI boot is really weird and customized, it’s not like
what you get from grub2-install --target=x86_64-efi. Looking
further into sources for this, looks like the additional files come
from a package named shim or the like, which is primarily designed
to support Secure Boot but also packs in some fixes for several UEFI
annoyances.
20201217/DuckDuckGo fallback.efi centos
20201217/https://www.rodsbooks.com/efi-bootloaders/fallback.html
And actually, I have to comment further. The only way to support
Secure Boot is to use shim. If you use grub2-install
--target=x86_64-efi, especially in conjunction with --removable,
Secure Boot will not work. This is because the Secure Boot shim will
be overwritten but the regular GRUB EFI bootloader that does not
support Secure Boot. So, point in hand, do not use the previous
commands to install GRUB if yoiu want to support Secure Boot. CentOS
has a special package that installs GRUB directly into the EFI System
Partition (ESP), it is named grub2-efi-x64.
64-bit:
sudo yum -y install grub2-efi-x64 shim-x64
32-bit (is it really any use?):
sudo yum -y install grub2-efi shim
Then, of course, finish up by updating your GRUB config, including copying the config files to the ESP.
rm /boot/grub2/grubenv # Ensure we don't symlink into the ESP.
grub2-mkconfig -o /boot/grub2/grub.cfg
cp -p /boot/grub2/{grub.cfg,grubenv} /boot/efi/EFI/centos/
Another thing of note from the referenced URL, tweaking
/boot/efi/EFI/centos/BOOTX64.CSV allows you to tweak the name of the
boot menu entry that is automatically added when you boot through
Shim. This is nice because when working with removable disks, you
cannot otherwise set the boot menu entry label text, except through
the fallback mechanism that automatically adds the boot menu entry.