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How to Buy a 2.5-inch Internal Hard Disk Drive

Hard disk drives (or HDDs) are often the largest storage space inside of a computer. Magnetic heads read and write data onto platters, which are separated into sectors. Laptops that use HDDs as their storage type require a smaller 2.5-inch size, rather than the 3.5-inch format used in desktops.

How does a hard disk drive work?

An internal hard drive uses magnets to read and write data onto the rotating platters inside the housing.

  • Billions of areas on the platter can be magnetized to a "1" or demagnetized to a "0". The data that is written onto the drive's platters is stored even when there is no power, with the platters being made of a hard material that is coated in a magnetic metal.
  • Two heads are assigned to each platter: One reads the top and the other reads the bottom. An arm moves the heads from the center of the platter to the edge, and then back again as needed. The heads do not touch the disk during this process.
  • Circular tracks are divided into sectors, and an area of storage stores a "map" that can be referenced regarding which sectors are full or empty. The controller then selects tracks and converts data being read and written, which is sent to the computer with a SATA cable.

What features do hard drives have?

Capacity, read and write speed, and size are important factors to consider when shopping for a new storage device.

  • Capacity is measured in gigabytes or terabytes (GB or TB). 250 GB is considered adequate for average users, while 500 GB to 1 TB is usually enough for those storing movies or games. The advertised capacity differs from the actual due to conversion, thus an 80 GB drive, for example, will only have around 74 GB available. The operating system and reserved space take up some room as well.
  • Read and write speed is measured in how much data can be handled per second. 500 MB/s would mean the hard drive can handle 500 MB of data in one second. The SATA interface also has limits, measured in Gbps. The rotation speed influences the rate at which different sectors can be selected.
  • Size differs between HDDs for desktops and laptops. A desktop drive is usually 3.5 inches, but smaller sizes can be used with adapters. Conversely, laptops using 2.5-inch drives cannot accommodate larger ones.

What drives are available for laptops?

2.5-inch drives are available in varied storage sizes, but their physical size must be the same.

  • Dell 1 TB internal hard drives have a rotation speed of 7,200 RPM, but HDDs are limited to around 100 MB/s read-write speed.
  • Some drives for laptops are sold with the trays or adapters needed to fit specific laptop models, so a Dell drive may be sold with a tray that fits into a laptop from that manufacturer.
  • Not all laptops allow users to replace internal hard drives without trouble, though upgrading to an SSD (solid-state drive) is another option.