GRand Unified Bootloader (GRUB) is a common boot tool used in Linux distros. It handles loading Linux and other operating systems. Apparently you don't really need it anymore, but I'm past the point of wanting to fuck too much with my computer if it's working.

I've come across hard disk trouble a few times that were solved by reinstalling GRUB. For example on booting getting a GRUB prompt with a no such partition error.

This has typically had to be done via booting with the Arch Linux install USB because it was impossible to boot otherwise. The process is the same as the initial install.

The process is:

1. Boot into the Arch install environment (make sure you're in UEFI mode if your computer supports it. All the following instructions assume UEFI)
2. Use lsblk to get locations of disks.
3. Mount those under /mnt and /mnt/boot as applicable.
4. Use arch-chroot to get into the mounted stuff and install GRUB as normal, eg:
   1. grub-install –target=x86_64-efi –efi-directory=/boot –bootloader-id=grub
   2. grub-mkconfig -o /boot/grub/grub.cfg
5. Reboot, cross your fingers.