I have an internal HDD with all my Steam games loaded on it. Everytime I boot my pc I have to open the HDD in the File Manager. Otherwise Steam does not recognise it and I have to manually add the Steam library back again. Is there a good fix for that?
Lacking details on your setup, I can only give general guidance.
All you have to do is edit (as root)
/etc/fstab and add the partition you wish to mount. For details, options, etc, check out https://help.ubuntu.com/community/AutomaticallyMountPartitions