You can mount or unmount using terminal.
Mounting with read andf write permissions:
sudo mount -o rw /dev/sdc /mnt
To unmount it, this is not a typo - The command is umount:
sudo umount /dev/sdc
If the manufacturer failed to include SMART controls, the Gnome Disks app will be limited in its ability to communicate with the device - but the terminal can do it just fine.
With the drive unmounted, run FileSystem Check on the drive:
sudo fsck /dev/sdc