1. Make entry in /etc/fstab.
2. Mount it whenever you want.
3. Set Steam Library to use the new drive.
4. Profit.

The fstab change is the important part,if you don't automount at startup, steam won't see the drive when you open it & the games will be listed as not installed.

It's easy enough do.

I see you use Mint, if you want to avoid the terminal, this method will automate the changes you need to make to fstab:

1. applications menu, type 'disks' & open it,
2. select the partition you want to mount,
3. click the options button- the 2 little cogs, select 'edit mount options',
4. hit the button at the top that says 'automatic mount options on/off', this should change the greyed out text to coloured. You should be able to leave the settings in here as they are. Take note of the Mount point listed, as you will need this for steam. Alternatively, you can just change it- mine is at /mnt/games

Then you just add your new steam library- the drive will be in the location you specified in point 4. create a new steam folder & you're done :)

Using terminal:
$ sudo blkid
#this will show you the UUID of your partitions. Find the one you want to mount- it will look something like: c97w43b4-7435-45ba-a677-fqga77f08ca3

$ sudo nano (or a text editor of your choice) etc/fstab

#add a line to the bottom of the file that you just opened.
UUID=(add your uuid here) /mnt/games(or location of where you want to mount the drive) ext4(or type if different) auto(or list the boot options you want to enable) 0 0

so the entry should look like:
UUID=c97w43b4-7435-45ba-a677-fqga77f08ca3 /mnt/games ext4 auto 0 0

Save the changes, reboot & see if it worked.

If it won't boot & drops you to a command line, don't panic. You can undo the changes easily. Just run the text editor again & delete/modify your changes to fstab, reboot & you should be back to normal.

Hope that helps. Let me know if I missed anything, or if anyone has a better way :)

Wait, before I read your replies, I just realized that I have not updated my profile. I am now using a laptop with an i5 and a gtx 960m and I am running KDE Neon.
KDE Neon is not selectable in hardware profile, admins please fix.

There is no disks application in Neon and please make it easy for me to do, I am not very knowledgeable. OK, wait, just installed that app from discover store. It's already set to mount on startup, I think i already did that in gparted.

What about permissions? It won't let me change any permissions or create a folder there in dolphin.

Do I make it a new partition or something? I thought I already did that. It's dev/sdb1

Thank you for your effort.^
The first problem was that properties was not set as a toolbar in dolphin browser. After placing the properties toolbar there I just typed sudo dolphin in terminal. It seems to go crazy outputting all kinds of stuff (looked like it was all about the dolphin emulator files) and then seems like it is pausing to output a bunch more stuff but I just closed it because I got what I want. Let me reread this thread as I just put that group has full permissions and then placed my name as group. Then I closed and reopened steam and tried to set the 2tb drive as an additional drive and steam did not see it.
Checking the disks application the little icons on it are broken. Mousing over them gave enough clues to find the spot you were talking about. It shows it is already set to mount at start up.

Where is the drive mounted?

Default is usually somewhere like /mnt or /media

You will find the drive in that folder. It doesn't show up as a d: or anything like in windows.

OK guys I got it now. In the disks app it appeared that it was set to mount at startup as that button was filled in solid and it even gave me a location of where it was mounted (remember the gifs are broken in that app) but going into the steam ui it wasn't seeing it so I went back to the disks app and yeah I chose mount it now and it mounted it. Seeing as how that was not how it appeared (there was no slider button saying mounted? yes no) I went back into automatic mount options thinking this is probably misleading also and I see an unchecked button there that says "show in UI."
It appears to be working now and I have added it as the default folder for steam. Thanks for all your help guys and yeah I was looking for it somewhere at the top of the folder tree like in windows.