Estos son los pasos. Es una copia de la Terminal:
[manjaro@manjaro ~]$ sudo mv /mnt/etc/grub.d /mnt/etc/grub.d.old
[manjaro@manjaro ~]$ manjaro-chroot -a
==> Detected systems:
--> 0) ManjaroLinux
==> Select system to mount [0-0] :
1
==> Mounting (ManjaroLinux) [/dev/sda10]
--> mount: [/mnt]
mount: /mnt: /dev/sda10 already mounted on /mnt.
--> mount: [/mnt/boot/efi]
--> mount: [/mnt/home]
[manjaro /]# sudo pacman -S grub
warning: grub-2.04-11.1 is up to date -- reinstalling
resolving dependencies...
looking for conflicting packages...
Packages (1) grub-2.04-11.1
Total Download Size: 9.88 MiB
Total Installed Size: 48.87 MiB
Net Upgrade Size: 0.00 MiB
:: Proceed with installation? [Y/n] Y
:: Retrieving packages...
grub-2.04-11.1-x... 9.9 MiB 1068 KiB/s 00:09 [###################] 100%
(1/1) checking keys in keyring [###################] 100%
(1/1) checking package integrity [###################] 100%
(1/1) loading package files [###################] 100%
(1/1) checking for file conflicts [###################] 100%
(1/1) checking available disk space [###################] 100%
warning: could not get file information for etc/grub.d/
warning: could not get file information for etc/grub.d/00_header
warning: could not get file information for etc/grub.d/10_linux
warning: could not get file information for etc/grub.d/20_linux_xen
warning: could not get file information for etc/grub.d/30_os-prober
warning: could not get file information for etc/grub.d/30_uefi-firmware
warning: could not get file information for etc/grub.d/40_custom
warning: could not get file information for etc/grub.d/41_custom
warning: could not get file information for etc/grub.d/README
:: Processing package changes...
(1/1) reinstalling grub [###################] 100%
:: Running post-transaction hooks...
(1/2) Arming ConditionNeedsUpdate...
(2/2) Updating the info directory file...
[manjaro /]# sudo update-grub
Generating grub configuration file ...
Found theme: /boot/grub/themes/starfield/theme.txt
Found linux image: /boot/vmlinuz-5.8-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.8-x86_64.img
Found initrd fallback image: /boot/initramfs-5.8-x86_64-fallback.img
Found linux image: /boot/vmlinuz-5.7-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.7-x86_64.img
Found initrd fallback image: /boot/initramfs-5.7-x86_64-fallback.img
Adding boot menu entry for UEFI Firmware Settings ...
done
[manjaro /]# exit
exit
--> umount: [/mnt/home]
--> umount: [/mnt/boot/efi]