Hello Joe - Looking at your partition table, it appears that sda4, the partition for LMDE, is not flagged as root. This probably occurred during the installation. You could try booting a LMDE live session from your installation media, then using Gparted to set the flag as root "/" on sda4, and then follow the commands in this link to re-install and update grub. http://howtoubuntu.org/how-to-repair-re ... tNCRJAo45s
Be sure you understand the commands, you want to mount and bind sda4, the root partition for LMDE, then reinstall grub to sda1, the boot partition, and finish with updating grub.