content: Would make a great nuclear-war-proof data backup method.  Not really, mechanical playback devices are lossy. It's like saving text as JPEG over and over again, except here it's written once (like cd-r) and every playback is like the JPEG's over and over again. Just ignoring the data degradation, assuming the data rate to be something like 20kbps or and assuming a length of 60 minute per cassette, each cassette would hold just about 9MB, or a little over 10MB for the 70 minute cassette. So, it would take almost seven minutes to store just a megabyte (assuming writing could be done as fast-slow as reading). You'd need 13 million of these cassettes to just store as much data as a 128GB uSD card, or probably twice or triple that, because it's lossy data and you'd need several copies for redundancy. So, to store the fingernail-sided SD-card nucrear-war-proof, you need to place it in a box sturdy enogh, let's say a 10x10x10cm steel cube. To store millions of these cartridges nuclear-proof, you'd need some underground bunker big enough to evacuate a small city.