> It also sounds like someone made the decision not to backup the raw images because they were “big” - that is actually the one thing they should have backed up because all of the smaller files can be regenerated from the raw ones.

Ironically my experience has been exactly the opposite. It's the demosaic-ed, fully developed copies of my photos that are larger and harder to preserve than the original RAWs. And these files can't be trivially regenerated from the RAWs, either. Let me explain.

The most important issue is that the process of taking a raw image and turning it into an edited RGB-pixel image is not obvious, at all. Tons of steps have to happen in this process, and there's currently no way to describe this process in a way that's compatible with a single open standard or even multiple pieces of software. At all. The steps can be broken down roughly into a series of "instructions", but what those instructions mean (e.g. to Lightroom) is entirely opaque and even secret in the case of closed source programs. Even open source programs, like RawTherapee and Darktable, use entirely different and incompatible approaches, algorithms, and instructions.

What this means on a practical level is that you can preserve your raw files and the instructions for replicating the edits as carefully as you like, but without the exact same piece of software (often even the same version of the program with the same settings and defaults set), your edits are as good as gone forever.

As a result I've had to take fairly drastic steps to make sure my photos are safe. For my older photos, I keep an entire VirtualBox image with Windows and Lightroom backed up along with my raw images, so that I can be sure of restoring exactly the same output files if necessary. (This more-or-less has to be a hacked version of Lightroom because you can't take chances with licensing problems preventing the program from running now that Lightroom is subscription based.) And actually for newer photos, I've moved away from Lightroom to RawTherapee. Even though I feel it's usually inferior, I feel safer since I know the pipeline from raw to burned edit is essentially public. I keep a backed up copy of the RawTherapee source code, but even if that somehow failed someone could make a RawTherapee compatible raw converter from scratch.

So that's why edited images are actually harder to preserve: whereas with raw you can just save them in 2-3 different physical locations and storage devices, to keep the edits you have to take several additional steps and there are more points of failure. Why not just keep the static edited images too? Well, I do that for my most essential photos. But that gets into the other issue, that the edited images are actually larger than the original raws.

If the point is preserving my edits, not just having a copy that's good enough for Facebook, the images have to be lossless. This pretty much means PNG or TIFF, and in fact the latter seems to be necessary since PNG doesn't handle metadata anywhere nearly as well. Unfortunately, while the compression used on raw files tends to be pretty good (which is further aided by the images being mosaiced), the compression algorithms compatible with TIFF are pretty terrible. Add that to the fact that you almost certainly want 16 bit images, in order to preserve as much raw detail as possible (in case you want high quality prints or need to do further editing), and you end up with whopping huge TIFFs. I regularly see my TIFF output files 4-5 times larger than the corresponding raws.

In short, managing backups for a high quality photography workflow is actually a good bit more difficult than it seems at first sight - and how it looks at first is not that easy either!