Due to a hardware failure I had to replace one of my computers (switching from a 2015 Intel NUC to a Dell OptiPlex Micro 7010).
After moving the disk to the new system, it refused to boot (claimed that no bootable drive was available).

Turns out that the new system only supports UEFI booting and the existing disk was setup for 'legacy'/CSM boot.

I used the following steps to convert the existing disk to UEFI boot (while keeping all data on it available).
They are inspired by the excellent Switch Debian from legacy to UEFI boot mode guide from Jens Getreu.

1. Disable secure boot in the BIOS to allow booting from an USB stick.
2. Create a bootable USB stick with a Debian live system (see my previous post)
3. Boot into the Debian live system
4. Identify the disk to work on (/dev/nvme0n1 in my case)
5. Convert the partition table from MBR to GPT:

   # gdisk /dev/nvme0n1
   
   r       recovery and transformation options (experts only)
   f       load MBR and build fresh GPT from it
   w	write table to disk and exit

6. Install gparted into the Debian live system:

   # apt-get install gparted

7. Create an UEFI partition and a partition for Grub2:

   # gparted /dev/nvme0n1

   Resize an existing partition to create space (does not need to be at the beginning of the disk, I used the swap partition).
   Create a new 100MB partition for efi (named "Efi partition"), format it as fat32 and flag it bootable.
   Create a new 50MB partition for Grub2 (named "BIOS boot partition"), keep it unformatted.
8. Use gdisk to set proper partition codes (EF00 for the efi partition and EF02 for the Grub2 partition):

   # gdisk /dev/nvme0n1
   
   p	print the partition table
   t	change a partition's type code
   t	change a partition's type code
   w	write table to disk and exit

9. Chroot into the on-disk root system:

   # mount -t ext4 /dev/nvme0n1p1 /mnt
   # mkdir /mnt/boot/efi
   # mount /dev/nvme0n1p2 /mnt/boot/efi
   # mount --bind /sys /mnt/sys
   # mount --bind /proc /mnt/proc
   # mount --bind /dev /mnt/dev
   # mount --bind /dev/pts /mnt/dev/pts
   # cp /etc/resolv.conf /mnt/etc/resolv.conf
   # chroot /mnt

10. Update /etc/fstab:

    # ls -lisa /dev/disk/by-uuid

    Identify the UUID of the EFI partition (usually in the format XXXX-XXXX) and add a corresponding line to /etc/fstab:

    # echo "UUID=XXXX-XXXX /boot/efi vfat defaults 0 2" >> /etc/fstab

11. Install grub-efi and install Grub2 to the EFI partition:

    # apt-get remove grub-pc
    # apt-get install grub-efi

    # grub-install /dev/nvme0n1

12. Exit the chroot and reboot the system:

    # exit
    # reboot

13. Select the Debian bootloader (/EFI/debian/grubx64.efi) in the UEFI BIOS and make it the default :-)