It is a Serial ATA hard drive with a 'built in' USB interface. The USB interface is part of the normal hard drive circuitry.
The downside to this means that when the interface fails you cannot use the hard drive again.
Whereas before you could disconnect the 'non working' USB interface and use the hard drive again in a laptop, 2.5 inch, or PC, 2.5/3.5 inch.
It's a marketing ploy I guess.