I am currently keeping my GOG installers (only English Windows versions for now, maybe Linux versions at some point) divided onto two separate 2TB hard drives (the combined size of the installers is currently something like 3TB or so, I figure). I keep the games starting with #-N on one hard drive, and the rest (O-Z) on the other.

So yeah you don't necessarily need to keep them all on one hard drive, if you have several smaller HDDs lying around. :) I guess I could buy an external 4TB HDD for them, but I guess I am too much of a cheapskate since this arrangement works for me currently.

EDIT: And when you say keeping them up to date is a real pain... apparently you were not aware of automatic tools like gogrepo? They automatically check what is new or changed, and download only those. It is not like I have manually downloaded each and every of my 1444 GOG game installer either, or constantly trying to keep track which of them receive any updates I should be downloading. No, I just run gogrepo and be done with it.

If it is data that i can't afford (or just don't want) to lose, then I simply keep one or more identical backups. For such files I have two 1:1 identical 3TB hard drives. I just ran a hash checksum for both of them two days ago (using rhash with sha256), all files on both were still fine. If I had detected some file was corrupted on one of the hard drives, I would have simply copied the uncorrupted copy over it from the other hard drive. I consider it too small of a possibility that exactly the same file(s) become corrupted on both hard drives around the same time. Yeah I know a meteor can hit my home too and I lose both hard drives, buuuut I figure at that point I would have bigger problems too, like being dead and all.

GOG games though, I am not too worried of losing because I do have "cloud backups" on GOG servers too. That is why I use only one single copies of them, mainly for easier access so that if I want to try some 10GB GOG game, I don't have to wait for the download first but just install it and be done with it (especially if I am somewhere with poor internet coverage).

If I'd lose my GOG installer HDDs right now... oh well, I guess I'd have redownload them all at some point. A nuisance, not critical. However, if GOG informed that they are closing the store (a bit like DotEmu did a year ago?), THEN I would make sure I have more 1:1 local copies of my GOG installers, just like I am doing with my DotEmu installers right now.