Help a curious mind out?
Do any fellow commentards know of any study that attempts to guess at the total amount of storage capacity the world has, broken down by media? I mean, there must be billions of SD cards, tens of billions of hard disks; CD's and DVDs could be looking at trillions - but do sheer numbers outweigh tape in terms of raw bit storage capacity?
I know there are different use cases and data lifetime plays a factor (how much of the data capacity on CDs is no longer readable?), but I imagine this is a question someone with better resources to hand may have asked.
Someone must have historical data for "units manufactured", and recent data will dwarf any inaccuracies from incomplete data from anything more than a few years ago.
Perhaps a suggestion for xkcd to look into...