When I have "too many" input devices plugged in, I cannot switch between operating systems in the GRUB menu. By unplugging devices and rebooting the computer, I can usually cause my keyboard to be recognized by GRUB so that I can switch options in the menu. However, which devices need to be unplugged, if any, is not deterministic.
Are there any diagnostics I can enable to see what keyboards GRUB detects?