https://www.amazon.com/SanDisk-256GB-microSDXC-Memory-Adapter/dp/B0758NHWS8
One hundred of those things packed into a 3 1/2 inch hard disk size, maybe? Of 25 terabytes? Add another $200 for the controller CPU, hardware,
256GB generic (probably OEM Adata's) USB 3.1 $28, work fine. 256GB generic SD 10uhs-1 is $33. 25TiB of either would be around $3100. 1TB of SATA SSD is $1000, 25TiB around $2800. Retail flash is at least 5x more $ than platters. That's all retail so a production run would be quite cheaper once you get a PCB run, some bulk trays of flash and junk, and an easybake oven to flow solder in house, seems pretty clear you could make a killing bidding the likes of whoever they were selling 25TB to at $40k. The chip makers will quote you trays without much fuss. Or try OEM the whole thing. Marketing a either 10x size, or speed, or both... will profit.
(Warning! Most 'off brand' micro-SD devices are FAKE)
Capacity fill test. Flash and hard drives do compression now, so to speed test you can't use /dev/zero, use /dev/urandom instead.
Something like that should be available already.
Haven't seen anything. Users with basic 10Mbps internet would take 255 days to fill one.