Backing up to an external drive and storing offsite certainly isn't a bad idea, but it is prone to human error and mechanical failure. The biggest drawback is that it requires someone to do something in order to make the backups, take them offsite, and do it regularly enough to catch any changes that are made/updated to make it effective.
Personally, I utilize a combination of strategies that may or may not work well for you. I use a Windows Home server box as my central storage repository. Within that I use folder duplication for my important data (like pictures, etc.) So that is two copies of the important stuff. I also utilize the built-in backup feature of WHS to make a copy of the important stuff to an external drive daily - 3rd Copy on another drive. I also have copy of all the pictures on the laptop that is actually used to view/download them from the camera, so that's a 4th copy. That's all Local, so it's only good if I'm fighting a drive failure, not some kind of natural disaster, theft, etc. For remote backup I use Crashplan. I used the free version for almost a year in my own little backup network between work/home/parent's house so it's very possible to set up something fairly robust for no cost, assuming you don't have to but more hard disks at todays inflated prices. You can designate a portion of your local storage as a backup location for your dad and vise versa if you want to take advantage of it too. You can even create an initial backup to an external drive and then bring it over to whereever the remote location is to speed up the initial backup process without taxing your internet connection. I used two remote locations in case one of them wasn't available for whatever reason I wasn't dead in the water. So that woudl be 6 locations total.
I could continue to wax philosophical about Crashplan as I really like it as a program/solution for backup - something that is essential but often a pain to do well (which typically means it doesn't get done). I mentioned I used the free version for a while but I also recently signed up for the 10 computer 4 year family package which gives you the lowest per month cost they offer for unlimited backup to their servers which would be a 7th copy. I believe they're also running a 10% off holiday sale right now as well.
I realize that is a little extreme and I would certainly think you could be reasonably assured your photos are safe with something like 3 copies, but I also know that i can't go back in time to take pictures of my kids growing up so the cost of storage is of little consequence when considering the alternative. Another thing to cosider is that you can store the actual albums offsite with a DVD and Hard drive copy which would almost certainly assure you that you could recover that data in a worst case scenario.
If WHS or Crashplan don't do it for you, I've also used Microsoft folder sync (now called something else) to good effect and there is certainly the syncback to external drive to be stored offsite option but again, that requires attention and effert on someones part to do well.
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