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SED stands for Self Encrypting Drive. This means it will automatically encrypt the files on the drive without you needing to take any steps. Many, if not most current hard drives feature SED. Of course, it doesn’t really protect your data on it’s own since it automatically decrypts the data whenever it is accessed. However you can add a key that will keep the drive locked and encrypted.
HERE is a nice write-up on SEDs from Techspot that goes into greater detail.