We have 3 computers in the office where I am sitting now. Each hard drive is backed up to a tape and we use a system of 3 tapes for each hard drive (9 tapes total). Two tapes for each hard drive are kept on premises in case of loss of data or should that particular computer be infected with a virus, and one tape for each hard drive is kept in a safety deposit box off-site in case of severe disaster like a fire loss.
I have often been curious, if your computer is using a mirror drive and your hard drive becomes infected is the mirror drive affected also? If so there would be a real advantage to using a tape where you could in effect go back in time to a point where your hard drive was not infected..