By chance, I just discovered a thing called "M-Disc"
Not sure what cave I've been stuck in, but as often repeated on this forum for oh, about 8 years or so, you can quote me saying over and over "Hard drives are the only reasonable archival storage for RAW files"
U.S. Navy has tested and adopted them, relying on the 1,000 year archival rating.
I did not add an extra zero, yes Virginia, rated fro 1,000 years.
Capacity still a problem?
Not so much.
Originally Available in 4.5GB DVD size, they are also now available in 25gb, 100gb and soon 50gb Blu Ray Disk capacities.
Of course you need an M-Disk compatible writer, but most DVD and Blu ray readers should read the written disks.
Cost per MB and time taken to write to multiple disks to archive an entire multi TB RAW library suddenly becomes reasonable again, with 100GB disks and a promise of 1,000 year life span.
ie: unlike standard CD-R DVD-R etc. no more leap frogging and re-re-writing every few years. In this case, do it once and you should be good for generations. (maybe buy an extra drive and seal it in the same fire proof safe eh? ) Adding 50 or 100GB at a time as new raw files are created seems very reasonable.
Anyway, as I said I just found it. I was looking to buy a DVD/BD-R for my PC and came across them by accident. I've only just installed it, so no experience yet, but if all goes well, I will have another methid for archiving, on site and off site. It couldn't hurt?