Cost and availability are the two daemons in Arching.
Well, there isn't anything that is certain. Everything we do is a compromise to minimise un-certainty. One thing about backing up is retrieving the data again, will the interface be available when I need it? Using SSD disks for archiving is great if cost isn't a factor, but try to archive 20+TB of data to SSD with redundancy, unless you are Bill Gates it ain't going to happen. So cheaper alternatives will have to be found.
Question, how quick do you want to retrieve your data, that is already archived? The faster you want it the more expensive it becomes.
Currently, I can live with multiple hard drive back-ups using hard drive cradles, this offers a fast and inexpensive way of backing up and retrieving my images fast, especially with Digital Assset Management software, with disks labeled and stored appropriately. This also offers me a thumbnail image to view the images, again this catalog also needs to be backed up multiple times with different time-stamps, as if one version of the catalog fails I can quickly roll back to the previous version.
Using multiple hard drives, say 3 2TB disks, use three different brand disks, Seagate, WD and Hitachi (in no particular order), so to avoid getting a bad batch of disks where suddenly all drives from a specific set fails you, oops!
Someone did calculate the failure rate of 1, 2 and many disks.
Having an alternative backup solution to the above is a good choice, this is where your magnetic tape comes in-handy or similar devices. This can very well be a slower solution, but the fact that its a different media, gives it the opportunity to perhaps sustain different threads that the first back couldn't.
I have mentioned it above, Digital Asset Management, but i will do so again, if you can't find it, it isn't worth the space it takes up!!! So make sure you have a catalog or some way of finding what you or someone else is looking for some time later in the future. Imagine that person not know whats there, they still have to know how to find things, so write a little note about how you have structured your files.
This could be your retirement fund!!!