Off-site backup is the proper answer to your question. All this really depends on your own tolerance or comfort with the possibility of losing data. The rule of thumb is that there should be at least three different copies of your data, each in a different physical location. For each of them, there should be redundancy of some kind implemented to guard against hardware failure. Redundancy is typically achieved by using mirrored drives or by using RAID of some kind. Also, if you’d like to know, using RAID in which you can only lose one disk in the array is not typically considered a sufficient level of protection because of the possibility of a cascading drive failure during replacement of a failed disk. It should be at least two.