Can't write to mounted ext4 hard drive in Ubuntu 18.04

I just built a new computer. I'm running Ubuntu 18.04, and I'm not dual-booting.

Ubuntu is installed on my SSD, but I'd like to also use an internal HDD (connected via SATA to my motherboard) for additional storage. I've been following this set of instructions to set things up. The HDD is formatted as ext4 and automatically mounts at boot, but that's as far as I've got. I'm stuck on Step 4, moving folders from ~/home/ to the HDD and making symbolic links.

I can't drag folders from Home to the mounted HDD via the file explorer. I also cannot create new directories in the mounted HDD via the file explorer. When I check the permissions of the mounted HDD (via right-click->Properties->Permissions), it says "The permissions of 'drivename' could not be determined."

From doing some further research, I suspect this is a permissions issue; however, I have been unable to determine what specifically I need to do. How do I troubleshoot this and write to my internal HDD?

Edit: The contents of my /etc/fstab file are as follows:

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sdb1 during installation
UUID=b928c257-beab-4352-81d0-9ae12955bf96 /               ext4    errors=remount-ro 0       1
# /boot/efi was on /dev/sdb3 during installation
UUID=5B6B-1052  /boot/efi       vfat    umask=0077      0       1
# /home was on /dev/sdb4 during installation
UUID=8d372b78-f15c-4c85-b853-becfafa28d69 /home           ext4    defaults        0       2
# swap was on /dev/sdb2 during installation
UUID=1972e189-a0fe-448d-93db-c17028c913c8 none            swap    sw              0       0
/dev/disk/by-uuid/c0c74b6f-dedb-4a3e-b5d8-52cca8a0958e /mnt/stuff auto nosuid,nodev,nofail,x-gvfs-show,x-gvfs-name=Stuff 0 0

Edit 2: Here's the relevant entry from running lsblk -fm -e 7:

sda  ext4         c0c74b6f-dedb-4a3e-b5d8-52cca8a0958e /mnt/stuff   1.8T root  disk  brw-rw----

The HDD mount made by the gnome-disk-manager should be ok
I guess you just have to fix perms on HDD :
Open a terminal CTRL+ALT+T and enter the command below

sudo chown $USER:$USER /mnt/stuff

If this does not the trick, go next:

sudo nano /etc/fstab #(or sudo gedit /etc/fstab)
#
# Edit the last line
UUID=c0c74b6f-dedb-4a3e-b5d8-52cca8a0958e /mnt/stuff ext4 defaults        0       2

Save the file and change (take) ownership on HDD

# Remount the HDD
sudo umount -f /mnt/stuff
sudo mount -a
sudo chown $USER:$USER /mnt/stuff
# non recursive change of ownership should be enough, but
# if still cannot write on it, go for a (longer) recursive change of ownership
# sudo chown -R $USER:$USER /mnt/stuff
# -R is for recursive