I was thinking about that too. It's been hinted at (one argument against infinite storage is that they use multiple similar rings, but that might be purely aesthetic for partitioning the power into easy to control chunks).
I guess (assuming that their universe has similar physics to ours) that storage mechanism is just packing energy/information or imparting structure onto the underlying structure of the magic (the magic is doing no work to an untouched 'mind). So there will be an upper limit on the density of energy or structure you can fit into a certain amount of metal. Once above these limits, it's possible that the metal may become unstable, or the information stored may start to be less reliable. I wouldn't want to wear a 'mind that might explode...nor would an unreliable source of data.
Another possible way of augmenting the knowledge storage would be to add a level of indirection, in which the raw data is processed, an error detection datum added to the end, and then the data can be compressed somehow, then placed on the 'mind. What I mean is some sort of synaptic processor between your mind and the metal, which takes what you wish to store, calculates a value that can be used to check for inconsistencies in the data later, and then compress it like a zip file. And for retrieval we do the opposite: decompress it, calculate the error checking value and compare to the one stored, and pass the data to the brain if its correct.
*overthinks*