If you happen to have some free space on the drive already then this is easy, just create a new partition of at least 100M.
The nice thing about the EFI spec is that it must just be in the first 2.2TB of space so for most users, it means you can simply resize the last partition (downsize it by 100M) and then add an EFI partition at the end.
For example if you had this partition scheme:
/dev/sda1 = /
/dev/sda2 = swap
You could just downsize your swap partition or whatever the last partition is and then create a new partitiion as EFI/FAT32 at the end. UEFI will still find and boot it even if it is not the first partition.
You could use gparted to achieve the above or from any other LiveCD like Ubuntu/Mint you could boot and manipulate the partition table as you need.
Replace the xx with your info eg. if you had /dev/sda3 as your EFI partition then use mkfs.vfat /dev/sda3
apt install grub-efi
Remember the EFI boot loader calls for a partition type of vfat/fat32 and it can be at any location (eg. I have booted from ESP as partition#3).
mkdir -p /boot/efi
mount /dev/sda1 /boot/efi
Note that some platforms/devices/computers/laptops/servers will not boot if you don't use the --bootloader-id option and if you don't use the --no-uefi-secure-boot
Change /dev/sdx to your drive
If you are using secureboot do this:
grub-install --target=x86_64-efi /dev/sdx
Installing for x86_64-efi platform.
grub-install: warning: EFI variables are not supported on this system..
Installation finished. No error reported.
grub-install --target=x86_64-efi /dev/sdx --no-uefi-secure-boot --bootloader-id realtechtalk
The whole issue is that the grub efi binaries are hard coded to find the files in /EFI/debian for Debian for example or for Ubuntu it is /EFI/ubuntu. If you use any bootloader-id other than those things won't work unless they exist.
To have a custom bootloader-id you need the hardcoded/default -bootloader-id in EFI/ and then you can install your customer one. In essence --bootloader-id is broken as users have complained of for years.
This is usually caused by the issues mentioned in #4, especially installing without --bootloader-id a lot of implementations of EFI will not boot if you don't set the --bootloader-id.
If you use a bootloader-id other than the default for the grub built for your OS eg. debian, ubuntu it will probably not work unless you use --no-uefi-secure-boot
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