Geometry...
The problem with this is that capacity goes up according to the areal density whilst sequential read speed only goes up according to the linear density (and random read time hardly at all) unless something can be done about increasing spin speed and seek head movement. The potential in those two elements of performance are very limited.
The result is that this 7 x increase in capacity will be accompanied by an increase in toptal time to read/write the entire disk by a factor of about 2.6. You could be looking at something approaching 20 hours to do that so rebuilding a RAID set or the like could be a long, long job.
