Steam downloads folder disappears/Steam seems to lose access to secondary HDD

I installed a game to a secondary HDD and every time I open Steam after a restart of my computer the Downloads folder I added disappears from the Steam UI. If I try to add it back again I can’t access it unless I open the filesystem manually (using Nautilus). Once I have the drive opened in Nautilus then I can add the folder back to Steam and my game shows up installed and I can play it. How can I find out what is going on here?

You should add the hardisk to your /etc/fstab so it is mounted on boot.