This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
SCSI-2 has commands that support third-party copying of data; one initiator tells device A to copy to device B. I don't know of any devices actually using this. Separation of control & data paths is a popular topic these days; can somebody comment on whether or not SCSI-3 supports this? I don't think so. (SHMO)