How to fix/reinstall grub from debian media, e.g. rescue mode?

How can one invoke grub-install from the debian netinst image? Or, does one need the live cd. (Situation: two disks: one degraded btrfs raid 1, another a blank disk I can install a new OS on, but which I can't boot from due to mobo. Currently upon booting, a black screen says "[Reboot and Select proper Boot device or Insert Boot Media in selected Boot device](https://askubuntu.com/questions/304144/clean-install-of-ubuntu-wont-boot-reboot-and-select-proper-boot-device) "; which program is saying this? Is this a GRUB message I can't find in the sourcecode? What is it trying to say; is it "failing successfully" or failing at a certain point?) I started the 'rescue' mode, selected "launch shell" (no root partition) when prompted, it set up a shell, but the only command is called grub-installer (not grub-install) which doesn't seem to do anything. Thank you.