Whilst industry-standard portable external drives in the 2.5" form factor typically have either 2 or 3 platters, is there any reason why there could not be an external drive with 4 platters?
It would presumably be 15.5mm thick (drive without the cover) rather than the current 12.5mm for a 3-platter drive, but it would crank up maximum capacity to 2TB, based on 4x500GB platters. I work in media and the bigger the disk, the better it is. However I can't lug the 3.5" disks which are way too heavy and unwieldy with their power supplies.
So a 2.5" solution would be ideal, and I believe there would be a market for it particularly among people working in media (movies, digital photography, music etc.) who require tons of data space.