Platters per Disk.

Headline:

• 2.5 inch:
• 9.5mm = 1 or 2 platters
• 12.5mm = 2 or 3 platters
• 15mm = ? platters. Guess at least 3. 4 unlikely, compare to 3.5 inch density
• 3.5 inch
• 25.4mm = commonly 4. Max. 5 platters.

Why is this useful, interesting or important?
To compare capacity across form-factors and for future configuration/design possibilities.

Disk form-factors are related by an approximate halving of platter area between sizes:

8::5.25 inch, 5.25::3.5 inch, 3.5::2.5 inch, 2.5::1.8 inch, 1.8::1.3 inch, 1.3::1 inch...

What we (as outsiders) know, but only approximately, is the recording area per platter for the platter sizes.  We know there are at least 3 regions of disk platter, but not their ratios/sizes, and these will vary per form-factor/platter-size:

• motor/hub. The area of the inside 'torus' is small, not much is lost.
• recorded area
• outer 'ring' for landing and idling or "unloading" heads. Coated differently (plastic?) to not damage heads if they "skid" or come into contact with a surface (vs 'flying' on the aerodynamic air-cushion).

Research:

Chris Mellor, 12th September 2011 12:02 GMT, The Register, "Five Platters, 4TB".

Seagate has a 4TB GoFlex Desk external drive but this is a 5-platter
disk with 800GB platters.

IDC, 2009, report sponsored by Hewlett-Packard:

By 2010, the HDD industry is expected to increase the maximum number of platters per 2.5inch performance-optimized HDD from two to three,
enabling them to accelerate delivering a doubling of capacity per drive, and subsequently achieving 50% capacity increases per drive over a shorter time frame.

7th September 2011 06:00 GMT, The Register.

Oddly Hitachi GST is only shipping single-platter versions of these new drives, although it is saying they are the first ones in a new family, with their 569Gbit/in2 areal density. The announced but not yet shipping terabyte platter Barracuda had a 635Gbit/in2 areal density.

Sebastian Anthony on December 12, 2011 at 12:25 pm, Extreme Tech

Hitachi, seemingly in defiance of the weather gods, has launched the
world’s largest 3.5-inch hard drive:
 The monstrous 4TB Deskstar 5K.
 With a rotational speed of 5,900RPM,
 a 6Gbps SATA 3 interface,
 and the same 32MB of cache as
 its 2 and 3TB siblings,
 the 4TB model is basically the same beast
— just with four platters instead of two or three.
 The list price is around $345

Silverton Consulting, 13-Sep-2011:

shipping over 1TB/disk platter using 3.5″ platters shipping with 569Gb/sqin technology