Mint installer tries to install GRUB to /dev/sda instead of /dev/sdb

I don't necessarily need it to be GRUB, I just want *a* bootloader. I've done a lot of troubleshooting already, but here's what I found. The last thing before the installer dies on me, it says:

mint grub-installer: error: Running 'grub-install --force --target x86_64-efi "/dev/sda"' failed.
mint rtkit-daemon: Supervising 11 threads of 8 processes of 1 users.
/usr/lib/ubiquity/ubiquity/frontend/gtk_components/nmwidgets.py:20: Warning: Source ID 6749 was not found when attempting to remove it.
    GLib.source_remove(self.timeout_id)
Unable to install GRUB in /dev/sda/
Executing 'grub-install /dev/sda' failed. This is a fatal error.

Now keep in mind, all three partitions of Linux are on sdb6, sdb5, and sdb7 (extended sdb3) respectively. At no point did I ever specify the system to touch sda whatsoever, so I don't know why it's trying to install GRUB or anything there. So I tried a couple of things. Mounting my EFI partition and attempting to chroot it yields this error:

chroot: failed to run command ‘/bin/bash’: No such file or directory

Initially, GRUB rescue gave me this back:

grub rescue> set prefix=(hd0,msdos8)/boot/grub
grub rescue> insmod normal
error: file '/boot/grub/i386-pc/normal.mod' not found.
grub rescue> normal
Unknown command 'normal'.

After performing cp -r on the directory from lib, it gives me a different, equally unhelpful error:

grub rescue> set boot=(hd0,msdos7)
grub rescue> set prefix=(hd0,msdos7)/boot/grub)
grub rescue> insmod normal
error: unknown filesystem.

A buddy of mine suggested to just unplug what is currently sda, and see what it tries to do then (since, again, I never asked it to install anything to that drive). No clue if it'll work or not, but I'm trying everything, and I appreciate any further ideas.