Warning: Back up your data. Copy your data to another physical disk or USB drive. The following action can cause your data to be deleted.
Do this to view the use of hard disks.
$ fdisk -l
Look for the hard disk to be formatted.
$ mkfs.ext4 /dev/XXX
XXX can be sda, sda1, sdb, sdb1, sdc, sdc1, etc. This command above should format the hard disk.
I needed to format the hard disk because one of the hard disks stopped working and I needed to add a new hard disk.