As others have mentioned, this definitely sound like the partition is being mounted by your desktop environment instead of your system. You can either add it manually by editing /etc/fstab or use the Disks program (think it's Gparted?) that I'm pretty sure Mint ships with, select your drive then your partition and click on the cogs to edit mount options (you might have to unmount it first). Deselect Automatic Mount if it's enabled, make sure mount at startup is enabled and check if the mount point points to a correct path, clicking OK should add the entry into your fstab, you can check if it's there with "cat /etc/fstab" in your terminal.