Run lsblk and identify your drive in the form /dev/sdXY where X is a letter and Y is a number (like /dev/sda2).
Run sudo blkid and see the UUID of that /dev/sdXY drive you previously identified.
Run mkdir $HOME/SteamDrive [this creates a SteamDrive folder in your home directory and will be where you mount the drive]
Run sudo nano fstab (this opens up a text editor in the terminal) and add an entry below the others in this form: UUID=uuid-from-step-2 /home/user/SteamDrive ext4 defaults,noatime 0 2
Make sure uuid-from-step-2 is exactly the one you got earlier. Replace user with your username of course.
Press Ctrl+O and then Enter to save the file. Then press Ctrl+X to exit the text editor.
Run reboot or restart the computer as usual.
Open up the Terminal app.
Run sudo chown -R user $HOME/SteamDrive (replace user with your username).