HDD form factor (hard disk drive form factor) is the size or geometry of a data storage device equipped with one or more magnetic-coated spinning platters and one or more moving actuator arms with magnetic heads to read and write information.
The HDD form factor determines the storage device’s physical compatibility with the drive bays in a storage array or enclosure, server, portable computer or other computing system or consumer electronics product. Industry standards dictate options for the length, width and height of HDDs as well as the position and orientation of the host interface connector.
The most common form factors for HDDs in enterprise systems are 2.5-inch, also known as small form factor (SFF), and 3.5-inch, also known as large form factor (LFF). The 2.5-inch and 3.5-inch measurements represent the approximate diameter of the platter within the drive enclosures. Enterprise-class HDD enclosures, or boxes, typically have a standard length and width. They can vary in height, up to 15 mm for SFF and up to 26.1 mm for LFF.
The 3.5-inch desktop HDD form factor offers a range of height options, from 19.9 mm to 26.1 mm. The mobile 2.5-inch HDD form factor ships in heights ranging from 5 mm to 15 mm. Two of the most popular mobile form factors are single-platter 7 mm and dual-platter 9.5 mm.
Many solid-state drives (SSDs) are designed for the HDD form factor. SSDs that fit into the same slots as HDDs generally use the serial ATA (SATA) or serial-attached SCSI (SAS) interface to transfer data to and from the host computing system.