Serial Advanced Technology Attachment (SATA) is the descendant of Parallel ATA (PATA), a protocol used by storage devices. Serial ATA differs from Parallel ATA by rapidly sending bits along a narrow path, whereas PATA uses a wider path that doesn't send its bits as fast. SATA is capable of much faster speeds, and its hard drives come in two different sizes: 3.5 inches and 2.5 inches. Here are the main ways in which they differ.
Some Background Context
The size designation is not actually a measurement of the device's width or length. The "3.5 inches" refers to the drive's original intent, which was to contain "floppy" disks 3.5 inches in width. At that time, hard drives were physically much larger than they are now, but with time comes miniaturization. People find ways to make various bits of hardware smaller, which uses less material, which makes the product cheaper to produce.
Eventually, hard drives were small enough to fit inside the part of a computer case that had been originally been designed for 3.5-inch floppy disk drives. In practice, a standard "3.5-inch" hard drive is actually 4 inches wide, 5.75 inches long and 1 inch tall. The older 5.25-inch drive enclosures are in still in use, but they are generally only used for optical drives, flash card readers, or audio-video panels.
However, miniaturization marches onward. The motivation to make hard drives even smaller comes from laptop manufacturers. A smaller hard drive frees up room for other components in the area underneath your laptop's keyboard, where physical space is a precious commodity. If you can make the hard drive small enough, you can fit more than one into a laptop, which can be a great advantage for some users. The 2.5-inch hard drives have become the norm in this sector.
Heat, Noise and Power Consumption
At first, these drives maxed out at 5400 RPM (revolutions per minute, an indirect measure of performance), but the 7200 RPM usually associated with desktop hard drives has become more common. The slower hard drives will still conserve substantially more battery power, and produce less heat and noise. The 7200 RPM, 2.5-inch drives, in turn, require less electricity and produce less noise and heat than their 3.5-inch cousins. However, the 3.5-inch drives will always be superior in terms of storage space. Most might choose that, but 2.5-inch drives are still popular with desktop users who favor a quiet computer, or users who like to design mini-computers.
Although 2.5-inch drives are small, they're not quite small enough to fit in most portable media devices, like an iPod. While some iPods can be upgraded with higher-capacity flash drives, they are not compatible with SATA drives, and flash storage is becoming increasingly popular because it has no moving parts, eliminating the noise and wear and tear of the moving parts of a standard hard drive.
The 2.5-inch hard drives are popular for external storage--more for portable hard drives than backup storage, since the 2.5-drives can't hold as much data. A smaller drive means a smaller (and lighter) drive enclosure.
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