Here's our answer (kind of?):
"Seagate Technology has introduced the world's first hard disk drive (HDD) with 4TB capacity. The new FreeAgent GoFlex Desk drive, which is available directly from the company, is based on a 3.5" HDD with four 1TB platters.
"Seagate's latest 3.5" platters have areal density of 625Gb/s per square inch, which means that the company can fit 1TB of data onto just one platter and thus make relatively inexpensive 2TB and 3TB hard drives as well as to create HDDs with 4TB and eventually 5TB capacities, if needed for competitive reasons."