How can I automatically mount a second internal SSD at boot?

Check your distribution’s documentation first, since it may have a guide specifically for automatically mounting additional drives at boot. In general, though, configuring /etc/fstab is the reliable and portable solution. Avoid mounting it only through a file manager, because that may mount it only after you log in rather than during startup.

The standard way is to add the drive to /etc/fstab. First identify its filesystem UUID with a command such as lsblk -f, create a permanent mount point like /mnt/games or /home/yourname/Games, then add an fstab entry using the UUID. Using the UUID is safer than referring to the device as /dev/sdX, since device names can change between boots. Test the entry with mount -a before rebooting.

You can also use a disk-management utility, such as KDE Partition Manager or another Disks application, to select the SSD and enable mounting at system startup. It will generally create the appropriate fstab configuration for you. Make sure the drive is formatted and that the mount location has the permissions you need for Steam and your user account.