I have a 250 GB SSD as boot drive. My steam games installed on a 2nd 2 TB hard disk. After reinstalling Ubuntu, i installed steam with.
apt install steam
Steam does not show the games installed on the 2 TB HDD.
To get the games work, i renamed steamapps folder, them created a symlink to directory on the 2 TB HDD where games are installed.
Stop steam, then do the following. /mnt/data/SteamLibrary/steamapps is the directory where my games are installed. Change it to whatever directory where your games are installed.
cd ~/.steam/debian-installation/ mv steamapps steamapps-old ln -s /mnt/data/SteamLibrary/steamapps
Now start Steam, you will see the games.