Most notebook hard drives conform to the 2.5" form factor. The standard, which calls for platters 2.5" in diameter, comes in multiple thicknesses. 9.5-mm variants are the most common, 12.5-mm flavors have become popular for high-capacity external drives, and 7-mm models are increasingly finding their way into ultrabooks. Now, it looks like hard drive makers may be slimming down even further.
According to DigiTimes' sources in the supply chain, drive manufacturers are developing notebook models just 5 mm thick. The ultra-skinny drives are reportedly in the planning stages, so they probably won't be on the market for a while. Shaving a couple millimeters—or in this case nearly 30%—off the thickness of a notebook hard drive isn't the sort of change that can be made overnight.
Why not focus on the existing 1.8" hard drive form factor? Because at 8 mm thick, it's actually pudgier than the slimmest 2.5" drives on the market. Also, the smaller platter diameter of 1.8" drives translates to much lower storage capacities.
If slimmer 2.5" drives are indeed on the way, they'll likely end up in hybrid storage configurations. Intel's ultrabook standard has very specific requirements about boot and standby performance, and meeting them all but demands a small caching SSD (or an all-in-one hybrid drive with an integrated SSD cache). Even with prices falling, solid-state storage will almost certainly remain too expensive to be used exclusively in budget ultrabooks.
As consumers adapt to lower-capacity tablets and get more of their data from networked sources, be it the cloud or local NAS solutions, I can't help but wonder if the demand for notebook storage will wane. Then again, the higher capacities that notebook drives provide could be key in differentiating ultrabooks from tablet hybrids.