I have the same issue. The solution is brutally removing the USB stick so
that the device configuration is reviewed and the hard disk will be treated
as /dev/sda, not the USB.
http://www.rants.org/2014/09/09/debian_install_solving_grub_error/
"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."
