Mac computers come with a very good backup solution called Time Machine. Time Machine creates both data backups as well as system image backups. To achieve Gold or Platinum level backup prevention, two external drives need to be deployed, alternating between them. Time Machine does allow multiple backup devices to be defined and once defined, it will automatically figure out which one is online and use it. The one slightly annoying side effect of this approach is that after a couple of weeks, Time Machine will issue warning messages that a backup has not been performed for xx days. This message applies to the offline backup device and can therefore be ignored.
So, if you have been using Time Machine with one external drive, go buy a second drive, define it in Time Machine, and alternate among the two drives. If you haven’t used Time Machine at all, go buy two external drives and define them accordingly.