Hard disk drives (HDDs) are used for storing masses of data in your system. They're mechanical, and thus slower than SSDs, but have the benefit of coming in much larger capacities – you can now get 12TB in a single drive. Internal drives come in two main form factors: 3.5in, which you typically use in desktop PCs, and 2.5in, which are usually for laptops, though a range of convertor brackets are also available. Consumer drives typically connect to the system via a SATA cable whereas drives destined for servers sometimes use what is known as a SAS connection.
