Maybe around an order of magnitude more. The big difference is that square inch of tape might be 3-5 orders of magnitude cheaper to manufacture.

Those SD cards aren't close to a single layer, so it's not really equivalent.

Ad absurdum, stacking two Micro SD cards on top of one another hasn't just doubled your density even though surface area is unchanged.

Fair enough but chances are the tapes will have a much longer lifetime for storing data, which is the primary use case for this sort of thing.

The density of flash memory is competitive with magnetic tapes, but the retention time is too low, making flash memory completely unusable for archival storage, even if it would have been as cheap as magnetic tape.

In theory, write-once memory cards, using some kind of antifuses, could be designed to have a lifetime good enough for archival storage, but nobody has attempted to develop such a technology, because it is not clear if there would be a market for them.

Most people do not think far ahead in the future, so they do not care much about archival data storage, until it is too late and the information had already been lost.

> The density of flash memory is competitive with magnetic tapes, but the retention time is too low, making flash memory completely unusable for archival storage, even if it would have been as cheap as magnetic tape.

I disagree that it's unusable. You'd end up with a puck the size of a data tape that can archive a petabyte of data and needs to be plugged in to a 5 watt power supply for long term storage. That's not super onerous. Then consider that tapes need to be stored at exactly room temperature with 20-50 percent humidity, while this puck would barely care about environment at all. And you could plug it directly into a computer without a $5k drive. Honestly it sounds pretty good to me. We just need to drop the price of flash by a factor of 20 to make the scenario happen.

It probably references the density within the data tape world, which is significant as there could be other ways to achieve a higher total storage, but this is one of the major components here it seems