Some external HDDs (notably, WD Essentials) are known to be encrypted by the USB-SATA bridge present in their case:
Essentials drives are hardware encrypted. The USB-SATA bridge board is required to decrypt the data.
This is confirmed by users who are unable to access any data once the HDD is extracted from its external case and attached directly to SATA.
I don't quite understand what purpose this kind of encryption serves. If the drive is lost or stolen, so is (most probably) the external case, so data security is unaffected. If anything, this makes legitimate data recovery more difficult, if the said bridge board is damaged. Am I missing something?