Self-Encrypting USB drives
A common external disk contains a disk like those used in desktop computers. Coordination between the disk SATA-interface and the computer USB-interface is provided by a special chip, called USB-to-SATA bridge. Until there is no encrypting, all the components from different models and even different manufacturers are compatible and mutually exchangeable. In the last couple of years manufacturers have been taking care of protecting data from thefts along with the disk.
A common external disk can be connected to any computer and be read. To make reading difficult, manufacturers started to encrypt disk's content. To do this an additional chip was used which was unique for each disk. When you access the disk for the first time you need to provide a password and then data is decrypted on reads, but during the write data is encrypted. The dirve is unlocked by providing a password until the power shuts down, at which time the drive reverts to the locked state.
The encryption chip was built either in USB-to-SATA bridge or directly in a disk. Thus, some parts of the external drives become not mutually exchangeable. On the one hand if a password is unknown, the data cannot be read. On the other hand if a chip which provides encryption fails the password is no longer accepted and data is lost irreversibly.
Continue to External drive recovery reference.