I have a very simple system.
It's mostly based on a) having lots of books now and b) not actually caring as much as I used to.
In theory, I back up my computer once a week using Time Machine, and stick the second drive back in the safe. This covers most eventualities.
But the real beauty of it is that at my age I no longer expect to remember exactly which books I owned anyway. And I'm not really bothered if the occasional one goes missing.
I was talking to an older friend recently and said:
"Please stop me if I've told you this anecdote before"
To which he replied:
"Carry on. It doesn't matter - I won't remember it if you have...."
It getting that way with the books I read too. I can re-read the old favourites every couple of years and they'll seem partly familiar yet still fresh.... So it's become a case of Back-up, Shmack-up - does it really matter that much?
Thanks to full bookshelves, a reasonable computer system, and the wonders of encroaching senility I'll never run out of things to read and re-read. If I unknowingly ended up buying the same thing twice it's not a big deal. Let the young control freaks fret about keeping all their possessions in perfectly precise piles at their fingertips - I've abandoned the pretence that I know where everything is around here anyway.
That reminds me - today is Sunday. If I can just recall the combination to the safe I'll do that back up..... maybe soonish....