It's funny, msking these things is completly different physics from making hard drives, but it's sort of the seme problem. Solar energy is theoretically limited by the total power radiated by the Sun through an area at the distance of the Earth. The reason solar cell efficiency is 30 probably has something to do with both the nature of the material, and some basic principle like the Virial Theorem applied to light as an ideal gas of photons. This is bad limit, like the weight of a laptop. The limits in magnetic storage are much better, and are made possible because c is very large and there is no interference from isolated magnetic poles, meaning that tiny B-fields can be measured. Because iron has two kinds of magnetic interaction (para- and ferro-) and various combinations of elements sometimes results in one of these properties being very pronounced, it is possible to make huge leaps based on a new material and manufacturing technique.

In any case both are dealing with light and its limits.