I agree with the general line of discussion in this thread thus far that it is best not to measure the capacity of data stored on a TOS Enterprise data wafer in terms of "bytes", "terabytes", or even "exabytes". The only comparison that makes any sense would be that these cards are very vaguely similar to today's SD camera cards, only they would naturally have zillions of times better storage capacity, along with similar interface bandwidth to allow nearly instantaneous data transfer, and that each wafer could probably hold data for very long periods of time without any data loss due to degradation or environmental effects.
In "Court Martial", "Balance of Terror", "Friday's Child" "The Lights of Zetar" and "Wink of an Eye", (to name a few) we saw Starfleet personnel using these wafers to store (presumably) complete personnel bio files, detailed sensor data, and even super-high-def video recordings. So it is safe to say that, just as a modern laptop, tablet or SLR camera packs more computing power than the astronauts took with them on all of the Apollo moon missions combined, a single wafer could offer superior storage capacity, transferability and durability than any modern server farm could.
And a tricorder or a communicator could compute faster and more powerfully than any of today's supercomputers could. All rhetoric I know, but that's the only possible frame of reference. And if the human race is still prospering 300 years from now (barring any PLANET OF THE APES scenarios), whether we have faster-than-light spaceships or not, you can bet these musings about computers will hold true.