When backing up you may get a warning indicating that you are backing up to a hard drive.
Most of the time, when you need to restore data it is because something has happened to your computer and you cannot access your original data. If your backup is on the same hard drive as your data, you might lose access to both, rendering the backup useless. This is why we recommend backing up to a USB stick or a cloud drive like Dropbox.
To resolve this, follow the instructions in the Manually Backing Up Data article to ensure that the backup is directed to the correct location.