This post is for people who install Debian GNU/Linux and run into this error at the GRUB installation step:
Executing 'grub-install /dev/sda' failed. This is a fatal error ...
Solution: remove the USB stick from which you are installing Debian. Right now it’s /dev/sda and the target hard drive you’re trying to install to is /dev/sdb (during the installation process only). This has bitten me more than once.
Yes, seriously: the Debian installer apparently can’t figure out on its own that most likely the place you want to install the boot loader is on the huuuuuuuuge hard drive that you’ve been installing to all this time, not on the USB stick that you’ve been installing from.
I can totally understand how new research in AI would be needed to solve that problem.