Western Digital is manufacturing a significant number of their external hard drives with an encrypted hard drive controller. The main chip on the external hard drive SATA to USB 2.0 & 3.0 PCB adapter boards, performs a block level encryption on the data as it is written to and read from the hard drive itself. This encryption process is not configurable and is not typically advertised on their feature list, most users are completely unaware that this process is happening.
Western Digital and other drive companies usually only give their encryption secrets to one single data recovery company, who can then charge whatever they want, knowing they are the only ones that can recover it. Prosoft does not recommend using hard drives with encryption unless money is no object (when it comes to recovery of the data from those drives, should they fail).
This encryption typically becomes an issue for the end user when they suffer some type of electronics failure with the enclosure electronics board. Often times the hard drive itself is removed to access the data, but no data can be found or extracted. None of the standard data recovery utilities are able to detect or decrypt the data and often find a significant amount of fragmented and bogus data.
It is essential that the hard drive is not formatted, reinitialized or altered in any way if you are needing to recover data from the drive; this will only cause corruption to the data and makes it more difficult to extract the data with the correct encryption key and electronics.