There are some WD Passport Essential models that came with a non-standard proprietary interface. The interfacing electronics were built into the HDD.
Also some current models from both WD and Seagate have similar issues, the USB interface is being built into the HDD electronics. There is no SATA connector. If the interface fails, you are stuffed - because you cannot crack the case open and connect the HDD to a standard SATA interface to get the data out.
The older type enclosures with a SATA 2.5" HDD is the better option.