Just recently I read the book Seal Target Geronimo about the seal team operation that killed Osama bin Laden. Seal operators have this saying, "One is none and two is one". Which means when you are dealing with physical equipment you have to have a backup if you want a high probability of having one piece of that equipment is always working. On the operation against Bin Laden the Seals learned something new about backup because they lost a super secret stealth helicopter because two of it's critical avionics boxes failed at the same time just as they were leaving the Bin Laden compound. Luckily they had two other helicopters. What the Seals learned is that having two backups is sometimes essential when you are dealing with complex electronics. Commercial avionics systems (like the 747 and 767, 777) have 3 copies of critical components that operate in a voting arrangement where any device that is "odd man out" (fails) in the voting gets disconnected.
What does this have to do with real estate photography? Everything! If you are like me your whole digital life is on entirely on a few external hard drives which are very failure prone. The natural tendency is to think that if you have one backup you are safe. No you're not! My motto that, I learned from my Boeing Avionics Engineer friends that designed the avionics systems in the 747, is that a file is not saved until it's in three places. Or "Three is one and two and one are none!"
I believe that your primary backup should be on site but your secondary backup should be off-site. This is to protect against a local disaster of some sort (like fire, earthquake or flood) that effects the location where your primary and backup are located. This offsite backup is I admit kind of a nuisance. I keep my secondary backup at my son's house, seven miles from my residence. I'd feel better if it was across the US from my home. The world is quickly evolving towards a model where automatic cloud storage will eventually be able to support off-site storage. I think we are a ways off yet but many services are beginning to support really large (TB size backup).
Peter Krogh's, The Dam Book is a great book that goes in to all the considerations of managing your digital assets. Take the time to think through the way you are managing your digital assets because a mistake in this area can be devastating!