The majority of external hard drives are nothing more than a case, a USB-SATA adapter, and a standard 3.5″ or 2.5″ hard drive. Despite having this extra hardware, they sometimes actually cost less than their bare counterparts. Why might this be? External drives usually only carry a 1 year warranty, while the average desktop drive today might get 2 years of coverage.
Above is a 3TB Seagate Expansion external hard drive. It has a USB 3.0 interface, and accepts 12VDC for power. I bought this a few years ago for $10 less than a bare 3TB SATA desktop drive.
Western Digital released a line of external hard drives where, when you take it apart, you don’t find a normal drive inside. Instead, the control board is actually a USB interface, not SATA.
Out of curiosity, and because I might put this drive inside my server in the future, I decided to tear it apart to see what I was working with.