On a WD My Passport or My Book the key lives on the USB board inside the case, generated when the drive was made and kept nowhere else. Without a password the board hands the key over to anyone who plugs the drive in, which is why the encryption is invisible day to day; set a WD Security password and the key is locked behind it. Either way the drive read without its board is noise. A dead board is repaired, or its key material read out and applied while the drive is imaged; a thrown-away board is a drive whose data no one will read.
BitLocker To Go on Windows keeps its key in the volume, protected by a password or a recovery key of 48 digits that Windows offered to save when it was turned on: to the Microsoft account, to a file, to a printout. FileVault and an encrypted Time Machine use the account password or the recovery key the Mac showed once. Seagate Secure and LaCie's tools use the password you chose. All of them are unlocked on the image with what you supply, and none of them can be unlocked without it.
A drive that has failed and is encrypted has two things wrong in order. The mechanics or the board are dealt with first, the drive is imaged through its own board so that WD's layer is undone, and the software layer is unlocked on the image afterwards. Encryption adds a step to the job; it usually adds nothing to the cost, provided the key is to hand.