I would like to add another hard drive to my desktop, roughly equal in size to my main hard drive, the purpose of which is to backup (mirror) the original drive. With other OSs there are programs to mirror these images automatically either on an hourly, daily, weekly basis. Some even allow for instant mirroring--delete a file from the original and it is automatically deleted from the secondary. Does Linux have any such software? Gratis or commercial? I'm mainly interested in the latter.