External USB enclosure. The 3.5" HDDs need external power. The 2.5" HDDs can usually run off of USB power, though they'll also work in a 3.5" enclosure (with external power).
There are also SATA-to-USB adapters, which let you drop in a HDD (doesn't fully enclose them). These are great for techies who frequently need to check what's on a HDD without fully installing it in a computer or enclosure. But I wouldn't recommend for carrying a drive around.