As I understand it, Signum (nee Burstcoin) is indeed the Proof of Capacity blockchain thingy that was using hard drives to store data with no external value. This is related to but different from to Proof of Storage schemes likes Filecoin where, IIRC, the data being stored was data of extrinsic value like PDFs or GIFs or whatever. I think that's also related to "Proof of Space-Time", meaning not only did you write the received data to disk, but you've _still_ got it written to disk.
PoC, e.g., Signum = get paid for proving that you paid for storage rather than CPU/GPU/ASIC cycles
PoS, e.g., Filecoin = get paid for renting out your storage to those willing to pay in return for storing data
Thanks for this really interesting side-thread; I have learned a lot! I've been interested in distributed storage for a long time and while I've known about PeerTube and IPFS and the Fediverse for ages I haven't really played with them personally. I go back and forth between keeping TiBs of storage online, and turning everything off as a concession to keeping my overall electricity bill in check. But in general I like the idea of letting my private machines contribute to something greater than themselves. I will have to look into the ways in which I can contribute to these projects.