2.5" Form Factor

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2.5" Form Factor

2.5" form factor drives are the standard today for notebook computers (although not all notebooks use them, most do). Since the notebook market continues to grow by leaps and bounds, sales of 2.5" form factor drives have been increasing, on a percentage basis, faster than probably any other segment of the hard disk market overall. While older laptops originally used 3.5" drives, the move to 2.5" was done for several reasons that are very important to mobile PC users (you can also find related information on the reduction of platter sizes here):

Size Reduction: Smaller drives take up less space and allow for laptops to be reduced in size. This trend began with the first 2.5" drives and continues with the continuous reduction in the heights of 2.5" drives (see below) and also the creation of still-smaller form factors.
Power Reduction: Smaller drives use less power, important for PCs that run on batteries.
Enhanced Rigidity: Smaller drives use smaller platters, which are less susceptible to damage as a result of shock, always a concern for a drive that will be moved around (often while operating!)

An 8.4 GB, 2.5" form factor IBM hard disk from my
notebook. Note the single connector in the front, which is
mated to a matching connector in the laptop's hard disk bay.
This allows the drive to be easily replaced at a later time.
The connector on the hard disk itself just uses straight pins
like a 3.5" hard disk form factor drive; the drive is
mounted into a carrier here, and the thin circuit board
you can see in the front "adapts" the regular pin connector
into the single Centronics-style connector my notebook uses.

Note: For more information on notebook hard disks, and a picture of a notebook drive's regular connector, see this page.

Unlike its larger, older siblings, the 2.5" form factor actually is named for the platter size of drives that use it (finally! :^) ) The width of a 2.5" drive is 2.75", and depth is 3.94". These drives originally came in just one height (0.75" or 19 mm). Since for any storage technology level there is a tradeoff between size and capacity, over time several different heights were created in this form factor as standards for mobile PC users with different requirements. They are usually specified in metric (mm) and to my knowledge have no fancy names:

Form Factor	Width (in)	Depth (in)	Height (in)	Application
2.5" 19 mm Height	2.75	3.94	0.75	Highest-capacity 2.5" drives, used in full-featured laptop systems
2.5" 17 mm Height	2.75	3.94	0.67	Mid-range capacity drives used in some laptop systems
2.5" 12.5 mm Height	2.75	3.94	0.49	Low-capacity drives used in small laptops (subnotebooks)
2.5" 9.5 mm Height	2.75	3.94	0.37	Lowest-capacity drives used in very small laptops (mini-subnotebooks)

2.5" drives are pretty much entrenched as the standard for laptop machines. They are also used occasionally in industrial applications, where the smaller size and increased ruggedness of portable drives is important.

Next: PC Card (PCMCIA) Form Factor

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