X-bit Labs notes the technology used in Hitachi's new Travelstar Z5K500 laptop HDD could be used to enable 3.5" desktop HDDs with 4TB or even 5TB capacities:
Hitachi Global Storage Technologies (Hitachi GST) on Thursday announced a series of new single-platter 2.5" hard disk drives (HDD) that are based on the sixth-generation perpendicular magnetic recording (PMR) platters. The latter feature higher areal density than currently available drives and can thus enable 3.5" hard drives with 4TB or even 5TB capacities.
Hitachi Travelstar Z5K500 hard disk drive comes in 2.5" form-factor and is just 7mm thick. The most important thing about this drive is that it is based on one 500GB sixth-generation PMR platter with 636Gb/inch² areal density. The platters themselves will eventually enable 2.5" drives with 1TB or 1.5TB capacities with two or three platters inside. Moreover, 3.5" platters with 636Gb/inch² areal density will have capacity of around 1TB, which means that it will be possible to create 4TB hard drives with four platters or even 5TB HDDs with five platters in the next twelve month, which is in line with expectations by analysts.