Coughlin Associates analyst Thomas M. Coughlin wrote in a report that the first 4TB HDDs should arrive by the end of next year:
Manufacturers of hard disk drives (HDDs) will continue to boost capacities of their products next year and by the end of 2011 hard drives with whopping 4TB capacities may emerge, according to an analyst. In order to build 4TB HDDs, manufacturers will have to create new platters with considerably higher than 530Gb/inch² areal density or make drives with five platters, like Hitachi does.
At present 3TB hard drive from Western Digital utilizes four 750GB platters with 530Gb/inch² areal density. A four platter drive with 4TB capacity will require four 1TB platters with even higher density, something that is not easy to achieve. Moreover, initially platters with higher areal density will be used in 2.5" or 2.5" external drives.