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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!)

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:

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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