1- Check the disk you want to mount:
sudo blkid
Then you can mount it using
mount /dev/sda2 /path/to/mountpoint
And add it to fstab
vim /etc/fstab
UUID=60ffd85b-6ab0-4eea-b752-727dd4e68eca /path/to/mountpoint ext4 defaults 0 2
Find sizes of all folders in a directory
du -sh *