This past week, Seagate finally announced a 6TB hard drive, which is three years after their 4TB hard drive. Of course, Hitachi announced their hermetically-sealed helium 6TB hard drives in November, 2013, but only to OEM and cloud customers, not for retail sale.
Hard drive capacities are slowing down as shown in the chart below. To account for the shrinking form factors in the earlier part of the history, and to account for exponential growth, I've scaled the vertical axis to be the logarithm of kilobytes (1000 bytes) per liter.
This three year drought on hard drive capacity increases is represented by the plateau between the last two blue dots in the graph, representing 2011 and 2014. The red line extension to 2020 is based on Seagate's prediction that by then they will have 20TB drives using HAMR technology, which uses a combination of laser and magnetism.
However, if the trendline from 2004-2011 had continued, by linear extrapolation on this log scale, hard drives would have been 600TB by 2020.
This is not good news for users of Big Data. Data sizes (and variety and number of sources) are continuing to grow, but hard drive sizes are leveling off. Horizontal scaling is no longer going to be an option; the days of the monolithic RDBMS are numbered. Worse, data center sizes and energy consumption will increase proportional to growth in data size rather than be tempered by advances in hard drive capacities as we had become accustomed to.
We haven't reached an absolute peak in hard drive capacity, so the term "peak hard drive" is an exaggeration in absolute terms, but relative to corporate data set sizes, I'm guessing we did reach peak hard drive a couple of years ago.