System Rescue Mode

If your system fails to boot, the "Enable/disable rescue mode" option lets you gain access to the hard disk. 

The rescue environment relies on Rescatux. After you enable it, boot up and then locate your original hard disk by typing the command

lsblk

This will show the disk devices. For instance, /dev/sda1 should match your hard drive's size (e.g., 50GB).

Next, mount that device to a folder like /mnt using the command

sudo mount /dev/sda1 /mnt

Make any necessary changes.

When you're done, disable rescue mode.