External hard drives are ideal for backup or portable file storage, and these drives come in a large range of categories: some use desktop-sized 3.5in hard disks, other use smaller 2.5in drives, and faster products use SSDs. Most drives use USB 3, Thunderbolt or eSATA connections, and these drives can also come in different types of case – some are designed to be tiny, and others are ruggedised to stand up to dust, water and even fire.