Physics News Update reports that current GMR (giant magnetoresistance) harddisk technology will not achieve terrabit-per-square-inch densities. Experiments with EMR (extra-ordinary magnetoresistive), which exceeds 100Gb/in^2 have been successful in the lab. There is even a diagram of the read head if you're interested ( http://www.aip.org/mgr/png/2002/153.htm ) http://www.aip.org/enews/physnews/2002/split/589-2.html