Coincidentally, my WD drive threw a delayed write failure error today, and I took it apart just to see what was inside. (Spot the engineer!) It's nothing more than a standard 2.5inch SATA drive with a little USB adaptor board mounted on the end, so it could be put into a drive dock of some sort after the adaptor was unplugged.

All the external drives I've looked inside have had a similar arrangement - there always seems to be a standard drive in there, with ATA or SATA connection according to vintage.