grub-reinstall

user@debian:~$ sudo modprobe efivarfs
user@debian:~$ sudo mount /dev/sda3 /mnt
user@debian:~$ sudo mount /dev/sda1 /mnt/boot/efi
user@debian:~$ for i in /dev /dev/pts /proc /sys /sys/firmware/efi/efivars /run; do sudo mount -B $i /mnt$i; done
user@debian:~$ sudo cp /etc/resolv.conf /mnt/etc/resolv.conf
user@debian:~$ sudo chroot /mnt
root@debian:/# apt update
Hit:1 http://deb.debian.org/debian bullseye InRelease
Get:2 http://security.debian.org/debian-security bullseye-security InRelease [44.1 kB]
Get:3 http://deb.debian.org/debian bullseye-updates InRelease [39.4 kB]
Fetched 83.5 kB in 1s (86.3 kB/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
74 packages can be upgraded. Run 'apt list --upgradable' to see them.
root@debian:/# mount -t efivarfs efivarfs /sys/firmware/efi/efivars
mount: /sys/firmware/efi/efivars: efivarfs already mounted on /sys/firmware/efi/efivars.
root@debian:/# apt install --reinstall grub-efi
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following package was automatically installed and is no longer required:
  sbsigntool
Use 'sudo apt autoremove' to remove it.
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 74 not upgraded.
Need to get 2,536 B of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://deb.debian.org/debian bullseye/main amd64 grub-efi amd64 2.04-20 [2,536 B]
Fetched 2,536 B in 0s (24.9 kB/s)
(Reading database ... 227056 files and directories currently installed.)
Preparing to unpack .../grub-efi_2.04-20_amd64.deb ...
Unpacking grub-efi (2.04-20) over (2.04-20) ...
Setting up grub-efi (2.04-20) ...
root@debian:/# grub-install /dev/sda
Installing for x86_64-efi platform.
Installation finished. No error reported.
root@debian:/# update-grub
Generating grub configuration file ...
Found background image: /usr/share/images/desktop-base/desktop-grub.png
Found linux image: /boot/vmlinuz-5.10.0-10-amd64
Found initrd image: /boot/initrd.img-5.10.0-10-amd64
Found linux image: /boot/vmlinuz-5.10.0-9-amd64
Found initrd image: /boot/initrd.img-5.10.0-9-amd64
grub-probe: error: cannot find a GRUB drive for /dev/sdb1.  Check your device.map.
Adding boot menu entry for EFI firmware configuration
done
root@debian:/# file /boot/efi/EFI/debian/grubx64.efi
/boot/efi/EFI/debian/grubx64.efi: PE32+ executable (EFI application) x86-64 (stripped to external PDB), for MS Windows
root@debian:/# efibootmgr -v | grep debian
Boot0000* debian	HD(1,GPT,c906677e-c2c2-42b5-b818-f0b19c046e95,0x800,0xfa000)/File(\EFI\debian\shimx64.efi)
root@debian:/# exit
exit
user@debian:~$ sudo chroot /mnt
root@debian:/# efibootmgr
BootCurrent: 000A
Timeout: 1 seconds
BootOrder: 0000,0003,0004,0008,0009,000A
Boot0000* debian
Boot0003* UEFI OS
Boot0004* UEFI OS
Boot0008* Onboard NIC(IPV4)
Boot0009* Onboard NIC(IPV6)
Boot000A* UEFI:  USB DISK 2.0 PMAP
root@debian:/# exit
exit
user@debian:~$ sudo umount /mnt/dev
umount: /mnt/dev: target is busy.
user@debian:~$ sudo umount /mnt/sys
umount: /mnt/sys: target is busy.
user@debian:~$ sudo umount /mnt/proc
user@debian:~$ sudo umount /mnt
umount: /mnt: target is busy.
user@debian:~$ sudo umount /mnt/boot/efi
user@debian:~$ sudo umount /mnt
umount: /mnt: target is busy.
user@debian:~$ sudo umount /mnt/dev/pts
user@debian:~$ sudo umount /mnt/sys/firmware/efi/efivars
user@debian:~$ sudo umount /mnt/run
user@debian:~$ sudo umount /mnt/dev
user@debian:~$ sudo umount /mnt/sys
user@debian:~$ sudo umount /mnt
user@debian:~$