Accessing the Rescue System

If your system fails to boot, turning on the "Enable/disable rescue mode" option gives you direct access to the hard disk. 

The rescue environment is built around Rescatux. Once you enable and boot into it, you'll first need to locate the original hard drive by typing:

lsblk

You should see device entries—something like /dev/sda1—that match your hard drive's capacity (e.g., 50 GB).

Next, mount that device to a folder such as /mnt with this command:

sudo mount /dev/sda1 /mnt

Now you can make any necessary changes to the files.

When you're done, don't forget to disable rescue mode again.