A box that has a tape drive and a hard disk. For every disk write my computer does, I want it to send to the box a copy of the data, what disk it was written to, and what sector number.
I want the box to store that information on its hard disk. When it accumulates enough that writing it to tape would not be too inefficient [1], it should write it out to tape and free the space on its hard disk.
I monitored my SSD writes for a few years, to see how long it would be before I had to worry about hitting their write limits. I found that I was writing under 3 TB/year on both my home and work machines.
A box like that, even with a modest size current tape cartridge, would be enough to handle all of my computers for several years before filling up the first cartridge.
[1] I don't know if this is still the case, but it used to be that when tape drives stopped and started, you'd get a gap that wasted some tape.