I'm curious to see how many years it takes for NAND flash storage to catch up on price.
As for as storage density goes, flash is way ahead. For example, it's possible to fit 6 x 6 x 6 (=216) MicroSD cards within the physical size of a 2.5" 7mm HDD. 64GB MicroSD cards are already on the market, so 216 of these would give 13.5TB.
As for standard 3.5" form-factor, 9 x 9 x 25 (=2025) MicroSD cards would fit, giving a potential capacity of 126.5TB.