Also amusing, since I got to poking around, using the benchmark
here for the current 1TB hard drive.
In fastest write speed from an external source it took 31 seconds to write 1GB of data. That means 31,000 seconds to write one terabyte; 31,000,000 seconds to write 1 petabyte; and 31,000,000,000 seconds to write on exabyte.
31,000,000,000 seconds equals about 982 years. So, not only do we need to wait for the exabyte hard drive, but we'll need to live a lot longer if we want to ever fill it. (I know, if such a thing ever came to be, write speeds would presumably have increased immensely as well).
Also from poking around, the current "pie in the sky" data density for hard drives is (I think) 1 terabit/in^2. That would be an 8,000 square inch hard drive just to get to a petabyte, and 8 million square inches for an exabyte. That would a hard drive 1.2 acres in size.
(And, of course, having wasted my time doing math with big numbers it will turn out that I forgot to carry a zero or something and everything is way off.)