Use GParted
If it is not in the menu (under gparted) then you can either install it from Software Manager (click on menu and type in software manager), or in Terminal.....:
Plug the external hard drive in.
Once you have opened GParted, in the top right hand corner there is a drop down menu which should show the various hard drives/ssd's etc..
Once you have selected the
correct drive (make sure!!!)...Click on Device, (menu item across the top) and then click on Create Partition Table. Choose GPT.
The right click in the main pane, or better still Select NEW (under Partition) and create/make a partition in EXT4 format.
In the section at top...there are GParted-Edit-View-Device-partition-Help......just below the word Partition, there is s
tick ...click on that, and you will be asked if you want to procedd with those actions and offers all sorts of dire warning re loss of data etc etc....click on OK
It will not take long...couple of minutes should see it done.
Exit from GParted. (just close it.)
Then, launch Disks (menu, type in disks)
From here we can
Edit the Mount options.
There is a small cog wheel just below the partition/disk you wish to mount at startup.
click on the cog wheel.....Then click on
Edit Mount Options.
Then toggle the 'Switch' (at the top) to OFF.......so that the disk will be mounted at startup
Click
OK in bottom right hand corner....this will exit disks, and it will also auto modify /etc/fstab so that the additional disk will be mounted every time you reboot.