9.5 Versus 12.5 mm: Which Notebook HDD Is Right For You?

Two- And Three-Platter Notebook Hard Drives

Notebook hard drive makers appear to have an ace up their collective sleeves. Today's most common notebook drives accommodate two rotating disks. It's possible, however, to create three-platter drives in a slightly taller form factor.

While those products could initially only operate at reduced spindle speeds (and hence, with reduced performance), the latest generation sheds this limitation, making three-platter drives equivalent to conventional two-platter models. This has ramifications in the entire 2.5” market.

See the difference? There's a clear z-height increase in moving from 9.5 mm to 12.5 mm drives.

Why Do I Care?

The technical term for this physical characteristic is the z-height. The x and y axes apply to the length and width of a hard drive. While the form factor determines the basic footprint and platter size, the z-height, in part, determines drive complexity. Most 2.5” notebook hard drives are based on a maximum z-height of 9.5 mm, but there are also models available that cap out at as little as 7 mm for ultra portable laptop designs (Seagate's Momentus Thin, for instance) and enterprise products stretching to 15 mm height (including the Hitachi Ultrastar C, Seagate’s Savvio, and Toshiba’s MBD, MBE, and MBF families). More z-height means space for more platters. More platters translates into higher capacities, but also into increased complexity.

12.5 mm: Why Now?

The 12.5 mm form factor has always been the odd man out. It doesn't provide sufficient space for multiple platters in enterprise drives, yet it's often too thick for laptops.

But the storage market is changing. More and more users are going for desktop replacement notebooks that, despite their name, can't fit 3.5” desktop drives. For external storage, portable drives have now replaced re-writable optical discs. Not least of all, devices like PVRs, set-top boxes, and nettop PCs require cost-effective storage solutions at sweet spot capacities, such as 1TB. All of this can be better served by triple-platter 2.5” drives.

What’s Next?

For the first time, 12.5 mm hard drives are now hitting high-capacity levels without introducing disadvantages to the end user. Samsung had a 500GB drive that crammed three platters into the 9.5 mm height, but apparently the approach had issues—otherwise we’d be seeing more drives of this kind.

All 2.5", 1TB disk drives from Toshiba and WD are based on the increased z-height, but they don’t impose any apparent compromises on the user. Older three-platter designs, such as the Fujitsu MHZ2500BT, operate at 4,200 RPM and hence had a clear performance disadvantage.

We decided to directly compare two of the latest 2.5” hard drives based on the same data density: a two-platter 640GB drive and a three-platter 1TB model. Is there a performance difference? Will the three platter drive require more power? We have the answers.