Why is it recommended that you do not back up your files to the same hard disk that windows is installed, explain in one sentence

It is recommended to not back up files to the same hard disk as Windows is installed because if the disk fails, both the Windows installation and the backup files would be lost simultaneously.

It is recommended not to back up files to the same hard disk as Windows because if the disk fails or gets corrupted, both the operating system and the backup files will be lost.