Every once in awhile I have a problem with Debian/Ubuntu installers running from USB thumb drives. What I think happens is the installer sees the USB drive first and the HDD second, so when it installs a bootloader, some information is written to the USB thumb drive that is needed to boot the computer. This is a problem because I don’t intend on leaving the USB thumb drive in the computer every time I need to boot it.
So my fix is quick and simple (and can be found all over the Internet). I only post it here so that I can easily find it in the future.
sudo grub-install /dev/sda # HDD device name