How to mount external hard drive EXT4 to ubuntu linux?

open nautilus check if drive appears - if it appears it has number like 68fdbd51-a4d0-4e10-b665-156f40dc2a86

check mounted devices - lsblk 

check fstab cat /etc/fstab

(replace username with Your system username)

sudo chown -R $USER:$USER /media/username/68fdbd51-a4d0-4e10-b665-156f40dc2a86

sudo chmod -R 777 /media/username/68fdbd51-a4d0-4e10-b665-156f40dc2a86