--I agree with you that clay or stone tablets stand the test of time. The problem becomes: how much data can you reliably store on such a thing, how thick should the stone be, how small should the bits be, where do you store it, and how conveniently can it be restored?
--M-discs are promising. They are basically stone-based Blu-ray DVD discs. Last I heard, they will have a 25GB version shipping as of March 20, 2014. I plan to get a set of 3 and burn the stuff that I've filed under NEVERLOSE - which really isn't all that much, it fits under 25GB. (I have multiple TB of JBODs at home but it's mostly VMs and a few movies.)
--The thing to do that most people don't think about, is PARE THE DATA DOWN. Does one average human being REALLY need 20TB of music, or pictures, or anything? Doubtless there are quite a bit of things in that pile that aren't in the top 25% of what you really enjoy the most.
--How do you even keep track of all that in your head? If you put all that music on random play, it would most likely go for more than a week or two. And I guarantee there would be songs in there that you wouldn't enjoy listening to that much - most albums are not 100% great.
--Separate out the stuff you enjoy less than ~65-75% of the time. Put it on secondary storage. Now you have less to back up.
--The only thing with current Blu-ray drives is that they're slow reading the data back. Linux is actually faster than Win7 when it comes to reading UDF discs on the same equipment, in my experience. Play around a bit, and get the data separated out so you can restore what you need the MOST in a certain period of time. YMMV.