Thanks for the suggestion, but cost is the stopper here. My short-term backup strategy is simply one of regularly copying precious data to a second disc so that, if the first disc crashes, I still have a copy. One of the problems with that is, if a file becomes corrupt and I fail to notice for a week or two, I'm just taking umpteen copies of the same corrupted file. And of course, if the house caught fire, it is likely that I'd lose the lot. And I certainly don't trust commercial backup software... it could be 'buggy' such that restored files don't work, or the vendor may not produce a version for (say) windows vista that restores files originally backed up on CP/M.
So, I also like to have a second strategy of taking plain, uncompressed and unoptimised
permanent copies of stuff whenever I get the urge. I can then go back a few days, a month, a year, or a decade or more, until I find a good copy of any affected file. At present, the cost of maintaining all that using SD cards seems excessive.
Since starting this thread, the price of Blu-Rays has about halved, although admittedly I couldn't find any (credible) source which had any in stock at these low prices, when I looked a day or two ago.