OK - lets take it from the beginning.
Locate which device id/partition matches your second device.
An educated guess on a system with two (2) disks is
/dev/sdb and with one partition it would be
Everything below assumes the device/partition to be
/dev/sdb1 - if it is not change the below commands to match accordingly.
Now you have identified your second device - lets get the UUID of the partition (sample output)
~ >>> lsblk -o UUID /dev/sdb1
Now select the identification - the long hexadecimal number on line number two in the output - with your mouse and press ShiftCtrlc.
Now edit the file
sudo nano /etc/fstab
Move to the line where you have
UUID=xxxx / ext4 and press Ctrlk then press Ctrlu twice
In the second line remove the UUID after the
= and press ShiftCtrlv to paste the UUID you copied before.
In the same line modify the root mount point
/ to read
Then press F2yEnter
Next create a new folder
sudo mkdir -p /data/steam
Now mount using fstab
sudo mount -a
To verify you actually mounted the partition (sample output)
~ >>> lsblk -la /dev/sdb
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdb 8:16 0 477G 0 disk
sdb1 8:17 0 400G 0 part /data/steam
Next change ownership on the content of the mount - ($USER translates to your logged in username)
sudo chmod -R $USER:$USER /data/steam
To verify you have writing access test by creating a file
echo "this is a test" > /data/steam/test.txt
Read the file back
~ >>> cat /data/steam/test.txt
this is a test
Then point your steam configurtion to use folder
/data/steam as installation target.